Looks to me like it is there to run your Mainsheet through to keep it up out of the way. I have a couple of velcro tie straps to hold mine up so I don't catch it on my neck when the boom is coming around. I havent ever seen one like that, usually just see some sort of strap.
Seems like that's a custom made fitting. Knowing now the location on the boom I was trying to visualize a purpose. Here's my 2 cents worth of guesses . . .