Kev, that answers my question and also confirms the length difference between the two plates. I guess the new plates never got given a NSN number.
Are there any other changes in NSN numbers between the early and later L4A1 magazines?
For both early and latter magazines they have the following NSN numbers:-
Early L4A1 Magazines
Case: 1005-99-960-2492
Platform: 1005-99-960-2794
Spring: 1005-99-960-0090
Retainer: 1005-99-960-3000
Bottom Plate: 1005-99-960-2044
Later L4A1 Magazines
Case: 1005-99-960-2492
Platform: 1005-99-960-2794
Spring: 1005-99-960-0090
Retainer: 1005-99-960-3000
Bottom Plate: 1005-99-961-7944