If you were looking at an original spruce boom,
I think you were looking at the outhaul mechanism on the outboard end. Mine is a threaded rod turned by a handle on the end. The car that attaches to the clew of the main runs on two SS plates as it travels the screw fore and aft.
The boom has a bronze sail track let into a gain cut along the top of the boom. Hope this helps, are you looking at a Challenger?