Last year I was looking at these fixtures while I was upgrading my under water zinc protection in the boat yard. They were terribly rusted. I took two pictures of them. I'm pretty sure that they are screws with square holes as shown below... but to be on the safe side wait for Kenny to respond.
If they are screws (as are the ones securing the transom mounted trim tab hinge plate) then be very careful as you unscrew them especially if they are rusted as in my photos because the rusted screw head could easily be twisted off. This then means drilling out the remaining piece of the screw and that would be a PITA. If it were me, I would remove all the screws and replace them with high quality SS ones with a larger size (larger diameter) so that if they rusted badly again there would be more metal left to avoid twisting the heads off.
You will also note that in my photos the brace plate attached to the transom is rusting badly also, so replacing the braces in my case isn't that far away for me.
[EDIT] I found some photos I took back in 2010 when I was having some Auto Pilot servicing. The photo below shows the backside of the transom where two of the brace
BOLTS can be clearly seen, side by side. They are immediately to the left of the large clear flex tube with black spiral wire reinforcement connected to grey ports for the self draining cockpit. They have washers under their securing nuts.
So it appears the brace is secured with bolts and NOT screws... and I would say they're at least 1/4" bolts and at least 1" long. The small screw underneath these two bolts is simply an attachment screw to secure a black shrouded wire bundle in place.
If you don't hear from Kenny I would get 1/4" x 1.5" long bolts/washers/nuts with the square holes for driving them. I would also play it safe and get backup bolts with a 3/8" dia. The current hole can always be enlarged if the old bolts have to be hammered or drilled out. Ensure the bolts have at least 1/4" of clear shank so there's more metal subject to rusting. The length can be longer than what the factory used as there's space for them to stick out in the area shown in my photo. I hope this helps you figure out what to buy.
