Thanks to everyone for the replies!
I bundled up and climbed out there to take another look. It does appear that they are 3/8 x 1 1/2...remembered them being smaller for some reason.
I filled the holes with epoxy a while back. In the spring I'll drill em out when its warmer and sealants will cure correctly.
Managed to snap a little pick of it.
My complete refit is taking completely too long!