Looks like an old(ish) open crab block. It lets you pull the marker buoy up and slip the rope over the pulley in the block and haul up the crab pot.
I think my granddad might have had one of these.
Normally they have an eye in one end to (the rh end in this case) to hang it from (above head height). (Now I can see the big photo it appears it does have an eye on the right side. So I’m sticking with crab/lobster pot block pulley.)
NANOTRAY 
Looks like an old(ish) open crab block. It lets you pull the marker buoy up and slip the rope over the pulley in the block and haul up the crab pot.
I think my granddad might have had one of these.
Normally they have an eye in one end to (the rh end in this case) to hang it from (above head height). (Now I can see the big photo it appears it does have an eye on the right side. So I’m sticking with crab/lobster pot block pulley.)
Am I close? Huh? Am I??