I've seen a few flattop upper receivers marked M4 above the gas tube hole and some had numbered rails and some didn't . Which one is supposed to be for the M4 ? Seems kinda silly to have 'em marked both ways.
Got to wonderin' if it was an option, specified by the buyer, gov't specs, or random pick (whichever receiver the assembly person grabbed first). I know, ITS A SALES GIMMICK! LOL