[Difference In Pre And Post Ban 1927?]

Have a friend who wants to sell me a pre ban 1927? Could someone tell me exactly what changes were made between the pre and the post ban 1927? Thanks for the help.

[1928 Finned Barrel On A M1a1?]

I wonder if someone could tell me if the 1928 finned barrel will work on the m1a1. I love the look of the finned barrel and was thinking about buying one. Thanks