30 vs 9mm AM III magazines?

Ok this is really strange. I tried shrinking the image to about 1/10th of its original size, I tried giving it more light, also making it a bit blue-ish, saving it as a bmp, jpg, png etc, and nothing worked. I try cutting off the guns, and it works from the first time…

So hereby the picture of my 5 .30carbine mags…

They all have the same type of follower. 

The previous owner of the first magazine has glued a little piece of white plastic where the follower catches the slidestop. I guess it has something to do with the angle of the follower. You can clearly see that the follower of the other 4 mags sits slightly higher in the front. Works perfect like this. No feeding issues with any of the mags. 

Well, this is odd for sure.....

Here are 5 mags.  One of the orange follower ones is Mike's, the other 3 orange followers are mine.  The one with the black follower is for my AMT-III 9mm WM.

I've been trying to figure this out with Mike.  Mike mentioned (and Mike, please correct me if i am wrong), that he has an AMT-III 30 Carbine.  Two of his mags fit and function great, and one "drags" as soon as it is inserted into in his magwell.  He mailed it to me for comparison.  So, I measured his with my three 30 carbine mags, and my 9mm WM mag.

Here are my numbers (below).  When I say "lip", that is measuring the back of the mag, the "U" shaped area (that Luc measured and posted above).  Note, my "lip" measurements are just the opposite of Luc's measurements.

I checked all 5 mags in my IAI 30 carbine.  They all insert with no "drag".  They all "fall out" into my hand when the mag release button is pressed.

I checked all 5 mags in my AMT 9mm WM.  Again, they all insert with no "drag".  They all "fall out" into my hand when the mag release button is pressed.

All 5 fit without drag in both my pistols.

Mike is having "drag" issues with 1 of his three mags (the one he sent me), and no drag issues with the other 2.  

With all this being said, is it possible, just maybe, that Mike's AMT III 30 Carbine is actually a 9mm WM frame with the 30 carbine upper?

Could my IAI 30 carbine, and AMT 9mm WM, both have 30 carbine frames and different uppers?  This would explain why the "larger" magazine "drags" in Mike's, and not in either of mine?

Mike and I both agree that it wouldn't make financial sense to actually cast 2 different frames to accommodate the 2 different calibers for a magwell difference of approximately 0.025".  (Sort of the same concept for the AMT-IV in 45 WIN MAG and 10MM Mag.  It's assumed that both of those frames are the same too, right?  In fact, isn't the AMT-V in 50 AE the same frame as the AMT-IV?)

Maybe in the beginning AMT had 2 different frames cast, then later (because of the whole AMT / IAI fiasco's) just went with the one "larger" frame and 2 different uppers to accommodate both caliber sizes?

Here is a 9mm WM round inserted in a presumed 30 carbine magazine.

The different dimensions in the mags make sense if you look at the dimension of the rounds.

The dimensions at the base of the 9mm is 0.391".  The 30 carbine has a base width of 0.356".

This would explain why a 9mm magazine has a larger "U" shaped area in the back, and the 30 carbine is smaller.

I hope all this makes sense......