I'm fairly sure its for a lens or a scope or something maybe an extended tube for the site? Not real sure, however to say you sparked my curiosity is a vast understatement I actually went to my truck and got my bow out at 130 AM to see what you were talking about I think its like having that extra piece on a rifle scope to block out some of the light that's coming in? I'm not sure however. Good question good post I hope you get a positive answer.
Ryan.