Hi RCS. I have never even seen those pouches. The military accepted full scale production of the simplified M1A1icon in 1942. It used the same 30 round mag as the earlier Thompson but it didn't accept the earlier 50 round drum mags.

PS - I have a demilled M1A1 and I cannot believe how heavy they are. Mine has an empty straight mag... A full 50 mag would be incredible.