If it's not a 28, that's good. SPare parts for your gun. It's probably in teh case, becasue it was considered a spare part for your rifle. This is a good sign, as it shows the previous owner may have upgraded other parts to real HK parts. (BW5's were notorious for bad extractor springs, and a lot of clone owners often fixed other issues by installing HK bolt parts.)
Just hit me that teh real HK part may already be installed in your gun, and that the one in the bag is the take off bobcat peice.