"That looks just like the ones
"That looks just like the ones by the main company that makes wheel locks for cars. The other main transom lock has a nut with a loose sleeve around it, which makes it really hard to remove, since the nut can't be grabbed by a pliers or wrench. I would call West Marine or other major supplier and see if they have a phone number or website for this company. McGard is one company but their current locks don't look the same as yours and they're the only brand carried by my supplier.
To me, it looks like a wheel lock and the previous owner may have just used one of them instead of buying one. If it's old enough, they may not have been available at the time. Does the trailer have wheel locks? Can you contact the previous owner to get the key? You might be able to file a couple of flat spots on it and use a Channel Lock pliers. PB Blaster is the best penetrant I have seen and you'll need something to get into the thread. Also, use a metal rod and pound on the end to break it free."