It is 2 selenium diodes.(its a Full-wave rectifier but not a full wave bridge rectifier) you can replace it with a silicon bridge rectifier. just connect the 2 yellow wires marked 'Y' to the AC terminals on the new bridge rectifier and the wire marked 'R' to the + on the new bridge. the negitive terminal is not used (it is not grounded in any way)

Thanks Beaky. That should make things nice and simple for me.
I wasn't real sure what to do about the ground.