The boat came with a well thought out A-frame set-up which uses the winch on the trailer for power to raise the mast. The feet of the A-frame connect to the chainplates. The apex of the A had a hinge and an eye bolt that the winch strap hooked into. The mast has permanent wire lines that attach to the A-frame with snap hooks.
I started to take the hinge apart before I remembered to take a photo, so below you see it already in two pieces, whereas the hinge was originally bolted to the L-shaped pieces that hold the eye bolt:
I found the setup awkward, as the pair of legs would not lie on the deck spread apart, and one leg would start to fall overboard when the first foot was being attached to its chainplate. Also, I wanted the legs to be easier to handle and store, and I thought that having them in two pieces instead of one big piece would be better.
After I took the fittings off the apex joint, I rounded the ends of the boards and attached an eye bolt to each. I was going to put the eye bolts off-centre (marks), but ended up centering them on the boards.
I raised the mast with it like this, and it worked fine except that the bottom block kept twisting and binding the line somewhat. So after I got it home again I re-rove the line so that the end that I'm pulling comes off the middle sheave. As of this writing, I haven't tried it out with this configuration to see if it fixes the twisting.
I had tried to move the boat on the trailer with this set, and the smaller bronze snap hook broke. So I took that as a warning and replaced the snap hooks with stainless shackles:
But, why the block and tackle in the first place? This is so I can raise the mast without the trailer winch—i.e., when the boat is in the water and doesn't have the trailer winch handy. The bottom block is shackled to the bronze stem piece on the boat, to which the forestay and jib attach. Being able to raise and lower the mast will be good for maintenance of the mast and its rigging, and for getting under bridges. Not something that will be done often, but something that needs to be done right when it is done.
Thanks to Starchy for helping me try out the new block & tackle arrangement.