OK, here's the scoop from a nearby boater--those you show don't have the button in the middle, most have a button that you have to push down. Yours doesn't. If it doesn't have the Merriman Trident it may be a different brand. Anyway, that slot in the top is something you're supposed to place a large flat blade screwdriver or the end of the handle in (if you've got the right handle) to use as a screwdriver. Turn and it unthreads and allows you to remove the winch drum. Then you should see the attachment bolts holding it to your boat. Depending on brand and age it will either have springs and pawls or a bar system. You can decide what you've got and what to do once you've got it apart.
I've found pawls for similar snubber winches via Minney's Yacht Surplus
in Newport Beach, CA (used) and they have various winches and parts there. You may also get things via www.rigrite.com
here in the states.
Hope that helps.
PS though you're interested in the winch rebuild, you might wish to re-bed it to the boat while you've got it opened up.