Hatch covers fish class
You seem to be getting the Corra class and fish class mixed up. All main deck hatches on the fish class were wire pulled slabs. The wire led through a block at the end of the hatch which when the weight came on it, it turned a locking bar releasing the lids. Each end was 3 slabs. First one went up, , andthe wire connected to the middle of the second two. The last slap rolled on a wheel each side. Had to put a chain on each side to hold open. Problem with crew was closing. Had to take the weight to release the chains then lower the lids. Especially with 60 tonne derrick the crew actually or very nearly pulled the lids right off by continuing to heave instead of lowering the lids.
I am struggling to remember the tweendeck hatches but feel sure they were wire pulled slabs as well.
Corra class main deck hatches dragged themselves along a chain. A series of McGregors no.1 and 5 tween decks were pontoons and can't quite remember if they were the upper or lower hatch in no.5 one deck was ermines.