It is serving thread Hink. Don't see any reason for it to be there Myself. Not ever seeing anything like this before Wasn't sure if it belonged there or not.
From the pic. in Your brochure I see that it has been added on. The Gentleman I got it from is quite the " tinkerer " I also got 3 Bear target bows that are covered with tape residue from all the things he tried over the years. Like sights, clickers and wrist slings mostly all home made.
Do You know the correct string length for the Varabow?