Ok, enough guess work. Time for some dry testing and product comparison. I went out and bought a 7.62x39 mag even though I don't have a stanag mag gun in that calibre to test this out. Curiousity got the better of me...
LAR .223 Mag on Left, XCR 7.62x39 Mag on Right. Both in original form.
Picture 1 thru 3... As new and assembled. The LAR mag is obviously shorter, and a little curved forward while the XCR is longer and box shaped. They share the same width and front to back dimensions.
Pictures 4 thru 7... Followers and Lips (proper term?)
Picture 8... "Exploded" Side by Side
While there is some difference in the front "legs of the followers (223 being long front and back, the 7.62 being short up front), there is little difference between the areas that the round seats to. It would appear that the only real difference between these two mags is the outside length of the mag and some very minute differences in the "hump" in the followers where the round sits. The hump on the XCR mag is a little shorter front to back, and a little taller in the mag but not by very much. All other areas of the mags are simliar in design. Even the springs for each mag have the same number of coils.
Potential problem with the VERY minor diffences in the followers.... The second to last 223 round wants to jump out of the 7.62x39 mag. Unknown what will happen when more rounds are seated into the mag as I only have two snap caps, but a double feed appears to be likely that a double feed will occur when down to the last 2 (or few) 223 rounds in the XCR 7.62x39 mag.
Function tests with live fire to follow. Will report back when complete, but this make take a couple of days due to scheduale and such.