I think the M15's all had "regular" blue but depending on who polished it at the factory, some are brighter than others..My 15-2 in blue(B on label) has a finish as nice as my bright blued M19-3(BT). Its hard to tell the differerance many times
I have one from '52 with the dull or satin finish, and I have a -1 that is bright, I have always suspected a refinish on it, but I see no evidence of it.