Old boom, new outhaul.

[L124C]

I'm either going to fix or replace the (external) outhaul on my old boom. I believe the boom was originally a furling boom.
I need to open the end cap to work on the inside, and hopefully install a internal outhaul system.

I've removed (drilled/dremeled) both bolts (one on each side) that held the cap on the boom. The bolt head in the center of the cap does not want to move. It doesn't seem to protrude into the boom.

If I look through a hole on top of the boom, I can see a sheave. Don't know if it was a prior outhaul, or part of the furling system.

Anyone know what the bolt might be for?