Maybe I should mention that there is a single stand-up block just aft of this (almost visible in the photo), which I have decided is the spinnaker downhaul. It's definitely too far aft (midway between the forestay and mast) to be a jib downhaul. It has a rolling furler anyway.
The boat isn't really designed to drop the mast in any simple way - certainly not single-handed.
I'm not totally certain it is NOT for a halyard, but here's what I'm thinking:
The only reason a block is ever placed forward of the mast is to somehow facilitate a line going from above or forward of the block to a point below or aft of the block. So, it serves to reason that the thing it goes to is intended to be either pretty much directly above the block or pretty much at the tip of the bow.
With the rolling furler and all associated rigging accounted for, it leads me to think either about the anchor or the spinnaker pole. It may be that someone installed it as a spinnaker downhaul, and the next owner just installed another block just aft and never bothered to remove the old one (possibly not knowing what it was for).
The thing that bugs me about this theory is that this is a seriously beefy piece of hardware. Seems like it was made for some really rough loads. It "looks" like an anchor thing. It's just that the anchor roller on the bow is angled so that it is about 20-30 degrees off from being inline with this block which is exactly on the center line.
The mystery continues ...
