I think it's his tagging mechanism. Maybe model/caliber/year on it. And it's not tape, they look to be held on with rubber bands.. so probably just tags.
His PE's/DE's don't have them, but he has paper labels rubber banded to the buttstock.
One way to tag them, I guess. Especially when you have one of everything.
Edit: Yep, found a couple pictures I could read. "K 303", "K 30/30", etc.
