this is on an early mercury v6 outboard. essentially an alpha with a long driveshaft. looks like a set screw. 3/8 or 7/16th thread and i guess i need a small 3 pin spanner socket. or is this more of a hammer-and-chisel type of removal method?

I dont think that is a prop retainer
I remove it with a punch or big screwdriver 🪛 and hammer 🔨

generally one should ask first and hammer later. but that may be the only option.

Well no body is stealing that prop. Maybe a lot of penetrating oil and needle nose pliers or 8 penny nails and vice grips? Let us know how it goes.

tried to find a 3 pin spanner socket, but all are big enough for trim cylinder ends or larger. thanks. sort-of ideas there...well, the big nut is bronze. dremel made short work of the set screw. got nut off, and now looks like i need to burn the hub and pull or cut the inner parti of the hub. wife asked "why do you have to drink to enjoy boating?" "I don't. I drink to kill the PAIN of boating!!"

Lol!!! I'm leaning towards the violence of the hammer and chisel too, just thought I would float an alternative. Still want to see the end result.

Is that just a set screw that just puts pressure on the prop? It almost looks like a modern retainer behind the prop nut...

Does the small screw even protrude into anything to lock it in place? If it does - just drill it out

I Know this kind of retainers from Older aftermarket wheels. they had them on the wheel caps for protection against stealing them. Older Borbet wheels had them on my Volkswagen Corrado years back. The 3 holes were slightly out of middle so that you needed the particular key for them. chisel and hammer did the job when i lost the key back then.

I never saw one until your pic, but i entered:
security 3 hole bolt
into google and a whole bunch show up.

Wow! before reading all the replies the first thing that popped into my head is some crazy machinist made that for himself! My dad who just retired from that trade came up with "creative" ways around things....once made his own floor safe that used a magnet to unlock it with. Looked like a service cap for old plumbing but no matter how hard you tired you couldn't get it open!