Ch113,
Main mag body is usually a two piece affair welded down the midway seam - similar to an AR mag.
Look at the back on the pictured mag - see how it is folded over and spot welded -
It would have required a different assembly process - possibly quicker, or requiring of less skilled labor than the two piece hidden weld.
It was a late war design, so perhaps the motivation was: time to assemble, cost and skill or materials required, all mitigators to the reasoning behind the late-war decision process.
Same reasoning changed the shape and look of the MP44 itself, simplified, by necessity.
I don't know for sure, but I would guess the reason we don't see many of these mags is the factory or machinery that made them in this fashion, was destroyed as opposed to falling into Allied hands.
Some circumspect info. but hopefully this inspires you to do your own research on why this mag is desirable to MP44 collectors.