DavecUK It looks like a mould for a base to something, or for holding something in 2 slots in the bottom of a not too deep rectangular recess.. The lettering is designed to be impressed onto the top surface, as it’s in mirror writing.
Elcarajillo Systemic You are correct, it is for casting weights for divers belts (snorkeling and diving ). When sea diving I would carry 40 +lbs in addition to a cylinder and other equipment to be neutrally buoyant. My cylinder would contain app 6 lbs of compressed air.
