How long have you been stamping the letter on the upper? I have a BM XM15 E2S A3 that is a year or 2 old, SN L179xxx, an there is no letter on either lug that I can find. It has the BM barrel (B MP 1/9 etc), and the grey coating on the inside of the recvr. Is there anything elst that proves that this is a genuine BM upper? The finish is a perfect match, and everything goes together perfectly, but you never know what a crooked dealer might do.