Where to begin?
References for the interior of the Mark III are definitely less common than for the 2pdr tanks, your best bet is the artwork in the Osprey New Vanguard (but don't bother much with the colour plate commentary as it doesn't match the illustrations
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)
Starting from the hull front:
Spare track stowage on the nose is dubious, so leave it off
The bolts on the sloping plate around the headlights should be countersunk screws flush with the plate, sanding time
Italeri put blackout hoods on both headlights, remove the one on the lefthand light
The most notorious error is in the shape of the front end of the trackguards, Italeri copied some bogus restoration on the APG tank, the trackguard should have a continuous slope instead of a step. PE sets will take care of this
PE sets can provide some bits and pieces for the driver's cab - the flap on the right side of the visor is a pistol port so it doesn't have to be open
For some unknown reason the parts for the trackguard stowage bins and air intake covers on the engine deck were mistakenly swapped around. The crowbars should be on the left side of the tank and the U-shaped clips for the tow rope on the right. Swapping the bins and intake covers to the opposite side from the instructions puts everything in the right place. You can also cut the butterfly nuts off the tools and reattach at random angles for a better look
If you feel like it, rivets can be added to the vertical sides of the air intake covers and also the hull sides behind the air filters - the devices on the rear end of the trackguards are the air filters BTW, not exhausts
The external fuel tank needs a fuel line added from the fitting on the hull rear, running downward at first, then bending to the left before feeding up into the top louvre on the hull rear.
Tow hook is of the type more commonly seen on earlier marks (but not totally wrong), so it can either be left off or replaced with a heavier-duty one like the Cromwell's (I cheated and used a similar one from a Sherman)
You can also add an exhaust fishtail centrally in the bottom louvre of the hull rear
One thing you don't need to replace are the moulded handles on the rear end of the engine deck, they drop down flush with the covers and are meant to look like that
Which covers the hull......
As for the turret:
The worst parts are the turret hatches which lack the opening pistons and stays to stop the flaps opening too far. See the Osprey artwork for what's needed. The instructions also have you put the inside handles in the wrong place
Look to a PE set for a radio which is really needed in the turret bustle
If you want a periscope, then pinch one from a Stalin tank - the Soviet MK4 periscope is a straight copy of the Vickers one (which is itself derived from the Polish Gundlach design)
Spotlight on the turret side is a bit naff, a headlight from the Italeri Elefant looks much better
Not exactly major changes, but worth doing. I've probably forgotten others as I'm doing this off the cuff.
David