Same as the original boom on my
Columbia 26.
You've got two
steel bails on the end. One is for the mainsheet and one is for the topping
lift. On mine, I don't have a real topping
lift, rather a cable that comes off the backstay with a hook. I don't know if this is original to the C26, but I actually like that better than a traditional line type topping lift.
The half circle with groove thingy is for your
outhaul. You should have a cleat farther forward on that side of the boom that the outhaul line
cleats to. This makes for a miserable outhaul, hard to get tight with the friction against that half circle thing and no mechanical advantage to light tightening and cleating. But it works ... sort of.