The two big studs mounted to the stringer. They have a rubber bushing at the bottom. I was thinking spare prop holders, but I think they are too close to the water tank? Anyone have a better answer?

look like spare prop holders to me.
mounted properly the props won't move.

+1

looks like they started to install a generator then stopped after the mounts

If that's a staggered 42 the genny would go in front of stbd engine.

Yeah i first thought generator too, but yes it would be in front of the starboard motor. I'll have to measure and see if spare props would fit.