I have Dragon's 1/72-scale kit of the AAVP7A1 with RAM/RS.
One of the things I notice is that it only has two antenna masts (located at the rear of the vehicle). Various reference photos show four antennas (two at the rear, and two more on tall posts half-way along the vehicle. Is this an upgrade feature related to a particular year, or is it to do with a vehicle's particular role?

A standard one usually has just the two antennas. More than 2 antennas is generally a Command Vehicle of some type.

The AAVP7A1 RAM/RS should have four antennas. The two in the rear have hull mounts while the pair found alongside are on stanchions. If you browse through images you'll notice that they aren't all mounted in the same place to boot because there are actually two positions on each side to run antenna cables through the hull. This modification was carried out in mid-late 1995 with the installation of SINCGARS radios. Prior to SINCGARS the P7A1s had 3 antennas, two in the rear, same place as current but the antennas were taller and thicker with larger mounts then there was a much thinner antenna inboard of the troop commanders cupola.
The USMC site has some excellent photos of modern AAVs in action.
http://www.marines.mil/Photos.aspx?igsearch=aav

There are four places antennas can be mounted, just like the M113 has 6-8 mounting points as options, but normally, only two antennas/radios are mounted.

Gino,
Each of AAVs in this photo have four antennas: http://media.dma.mil/2014/May/22/2000797020/-1/-1/0/140515-M-UF252-168.JPG. They cannot all be command vehicles...

They could be.

Maybe the USMC uses a different comms setup, but normally a vehicle only has two antennas/radios. You usually talk to your element (BN, Company, PLT, etc.) and one higher; so Bn and BDE, Company and Bn, PLT and Company, etc.

Daniel, can you shed some more light on it?

The photos look about equally divided between those with four and those with two. But awesome photos. Now we know why the Marines call 'em "tuna boats." They really can swim like nobody's business.

Essentially it is an antenna for each radio. The aft most two antennas go to the primary radio (transmit/receiver) and the forward two antennas go to the Aux radios (receive only). One stack of two radios is for the vehicle with a control box (for changing channels remotely) located in the turret position and the second stack is for the embarked infantry with a control box at the troop commander's station. All positions, driver, troop commander, vehicle commander, rear crewman can listen to all four radios and talk on the two through the "Charlie" boxes based on the switch position they select.
There is also an additional antenna mount at the aft port which is for the EPLRS system antenna, although some P7s and the newer C7s have a new overhanging box like antenna mounted in this position. I tinkered with the C7 a bit but them being mounted to P7s occurred after my time on active duty.
I was able to pluck a page from the Operator’s Manual that shows the radios and their antenna locations.

Thanks for the first-hand info Daniel !

Here's a link to Daniel's AAV pics on Primeportal :

http://www.primeportal.net/apc/daniel_attilio/amtracs/index.php?Page=1

H.P.

These must be MPF tractors, they are way to clean and pretty to be regulars in one of the battalions (CAB, 2, 3, 4). Tow ropes are white and aren't stained, no scratches scars or chips and it looks like the shackles are still zip tied and actually match the hull color and the road wheels aren't rusted yet. Whooo weeee, snap the pictures before they put some miles on and they gain some "character".

OK, I guess that makes sense. Two radios for the vehicle and two for the grunts on the ground. In the Army, a Bradley and its infantry are all on the same radio nets.

Are the grunts assigned to the AMTRAC Bn or are they separate units; INF Bn and AMTRAC Bn? If they are separate units, this makes even more sense since the IN Bn can be picked up by different AMTRAC units.

In the Army, the Brads and grunts are assigned to the same unit; ie, 1-15 Infantry Bn has Bradleys and the crews/dismounts all assigned to 1-15 IN.

The infantry, AmTrac and tank are separate Battalions that are task organized as the mission dictates which allows for tremendous flexibility. For exercises and operations all necessary skills/assets are attached to the infantry Battalion, they run the show. Infantry units I worked with that come to mind on my last deployment within a span of seven months were 3/4, 2/1, 3/1 (battalion/regiment) and there are probably couple more that I just don’t recall at the moment.

Combat Assault Battalion, formerly 1st AmTrac Bn, Okinawa Japan (3D Marine Division/III MEF)
2D Assault Amphibian Bn, Camp Lejeune, NC (2nd Marine Division/II MEF)
3D Assault Amphibian Bn, Camp Pendleton, CA (1st Marine Division/I MEF)
4TH Assault Amphibian Bn, TX, FL, MS (4th Marine Division) – Reserves

I copied this from MCWP 3-13 Employment of Amphibious Assault Vehicles, albeit an older version I do not believe too much has changed in the previous three or so years in regards to this information.

Organization
AA (Assault Amphibian) units are organized and equipped to land the surface assault elements of the LF (Landing Force) and their equipment in a single lift from assault shipping during amphibious operations to inland objectives and to conduct mech operations and related combat support in subsequent operations ashore. In addition, AA units can conduct shore-to-shore assault operations, riverine operations, sea control operations in littoral areas, and other missions in support of Marine air-ground task force (MAGTF) operations. AA units may be tasked- organized as a maneuver element for riverine operations and operations in littoral areas for the conduct of rear area security. These varied missions reflect the flexibility and operational utility of the AAV on the modern battlefield.
Assault Amphibian Battalion
The AA battalion is assigned to a Marine division. The battalion and/or its subordinate units are attached to or placed in support of a GCE commander to provide ship-to-shore lift of the surface assault elements of the LF. Once ashore, the battalion provides tactical mobility and communications to the supported force. The AA battalion augments the AA company’s organic logistic capability by providing personnel, medical, resupply, and overflow second and third echelon maintenance. Although primarily employed to mechanize the surface assault elements of a regimental landing team (RLT), AA battalion elements may be employed in a CSS role forward of the forward edge of the battle area (FEBA) or in the beach support area. The AA battalion has the command, staff, and resources necessary to plan and execute mech operations as a maneuver control headquarters when augmented with combat, combat support forces, and FSCC representatives (i.e., air officer, naval guns liaison officer, 81-millimeter mortar forward observers [FOs], and artillery FO). The I Marine Expeditionary Force (MEF) and II MEF each have a full AA battalion. The III MEF possesses one AA company that is part of the unit deployment program. This program is sourced from I MEF. Marine Corps Forces Reserve has one AA battalion (minus) consisting of two line companies and one headquarters and service (H&S) company.

I do miss tinkering on AmTracs but life goes on and today school devours most of my time, so there is not much free time for reflection on days of times past. Aggravatingly there is also no time to work on my models.