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Re: What's this called, and where can I get a thumbscrew to replace the screw?
That homedepot shackle has a working load that's half of what a Harken, Ronstan, Wichard, etc. marine shackle is rated.
The OP should take his mainsheet block in and find a shackle that fits. A decent chandlery should have a whole shelf of different shackles. What you'll find is that the pin diameter is standard size, like 3/16" or 1/4", that many manufactures will make multiple models in. But the width of the opening in the shackle, how long it is, the outside width, etc. are all going to vary a bit between the different 3/16" pin shackles. So take the block in so you can find one that fits.
There are also bow or anchor shackles that are rounded. No reason you can't use one of those.
The pins usually aren't slotted for a screwdriver like that. Usually there is a flat tab you can twist off with your fingers, but sometimes they get really tight and pliers are needed. Often there is a hole in the tab and you can poke something in it instead of pliers. The hole can also be used to tie the shackle closed.
Commonly the pins are completely removable like the one you have, but some have captive pins. The hole on the non-threaded side is too small for the threaded part to fit through, so the pin won't fall out when the shackle is opened.
A keyed pin headboard shackle from post #5 probably won't work in a mainsheet block. Your block probably has a post or tabs with a hole(s) in it, which the shackle pin fits through. The hole is probably sized to just fit a 3/16" or whatever pin. The key on the pin of the headboard won't fit through.
Ronstan, Harken, Wichard, and others make snap shackles that are designed to be attached to blocks. There is a pin on one side and a snap shackle on the other. They are kind of pricey. Like $50 instead of $5. But if you want to take your mainsheet block on and off a lot, it will very convenient!
Garhauer sells various blocks with snap shackles as part of the block. You might find one of these blocks to be comparable in price to buying just a snap shackle for the block you have.