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OK. some AR15 history.

When Colt brought out the civilian AR15, they wanted to please the Feds by making their civie rifle as hard as possible to convert to full auto fire.  So, among the other things they did. they milled out the underside of the bolt carrier at the rear, so the collar of the firing pin was exposed, and put that notch in the top of the hammer.  Thus if someone tried to get auto fire by simply removing the disconnector and using extremely soft primrered ammunition, the notch would catch on the collar of the firing pin and hang up the rifle.

Now despite what they'd like you to believe, the aftermarket makers have ALWAYS copied anything Colt does, so they started doing it too. So for years they milled out the underside of their carriers and notched the hammers.  Over the past few years though, after Colt started shipping ARs with full bottomed carriers and unnotched hammers, all of a sudden the aftermarket started doing it also.  Rock River I believe started the trend by introducing their "improved bolt carrier" (what a joke) but increasingly all the other makers are using fully shrouded carriers and unnotched hammers

If you mail order your internal parts these days you might get the unnotched piece or a left over notched job.  Me? I go to the gun show, look for Colt or Colt contractor marked M16 milsurp pieces, take 'em home and cut away the "M16" portion of the parts. That way, I know I've got a quality piece in my rifle, and don't have to worry about, "did I get a good part or is it defective"..

And yes even the top dog reputation outfits will sell you POS crap once in awhile.  I once got a mag release directly from Rock River that was cracked right out of the box; and I've seen pics on this site of Stag hammers that were total garbage.