Gary,
the photo is in Squadron/Signal publications M3 Half Track in action, number 34. The photo can be found on the top left of page 20.
One mistake in my statement is a forward firing mortar, which was incorrect. It is however an M4A1 with added boxes on the rear bumper shelf and the mortar is clearly on a raised I-beam pedestal judging by how much of it shows protruding from the crew compartment, versus how little protrudes on a standard M4
The caption below the picture says nothing regarding the unit but does explain what makes it an M4A1 and why. So without some major scratch building, the kit decals are fairly useless.
I missed your edit when posting that, so I will address your points.
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The only forward pointing mortars on the M4 were the 2nd AD modification.
Well, not quite true, the 2nd armored may be famous for the forward facing mortars and use of the skate rail, but the 3rd armored actually had their own style. They removed the sides, fuel tanks etc. switched them around and faced the mortar forward by leaving the I-beam raised platform in the middle of the crew compartment and placing the bipod directly behind the driver/gunner seats, this allowed the use of the rear door, while the 2nd armored's version did not. This can be found in two publications as well, Tankograds manual has an interior shot on page 22 which explains in detail what was done and the Zaloga book, " in combat " has a picture of one on page 47 showing the reversed side panels, with the access doors to the rear.
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The M4A1 had the mortar mounted higher as the feet of the bipod were on a plate mounted at the same height as the rear seats so the mortar could have more traverse.
Yes, exactly. I have an extensive collection of photos of this. The flat steel plate with holes in it was used by the 3rd armored behind the driver/gunner seats, unlike the 2nd armored who used a skate rail piece instead. Perhaps I will post some photos.
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The first decal option was an brand new early M4 that was photographed extensively somewhere. The serial number does look a different color than the "M4" stenciled in front of the drivers compartment.
Yes it is, I believe it's the pilot version or the next in line. It can be found in the Hunnicutt book, several photos actually. The numbers
ARE a different color, as I stated, it is light blue, which would fade into the olive drab at a distance so as not to stand out. This was a pre D-Day thing which was later changed to white, but those markings on the decal sheet should be light blue.