Hmm, I would think you would see some right after the M2 came into play and the adaption of the 30 round mag. I have seen modified 15 rounders with a piece sewn in. Other than WB series I have never seen a 30 round mag pouch. Giess I will have to add that to my list of things to get.
WB2 does show MILCOM PRODUCTS INC/ QT (CTM) 2889, 11/28/58.
I do not know if that is a date or date of contract but if so it would seem that might be latest date marked.