The "sawtoothed" bar on your buckles is intended to grip a cloth/fabric strap, not rubber, because those teeth tend to tear up rubber straps. If you closely examine the adjustment-buckles on the rubber straps at the Links posted above, you'll see those buckles are not the "toothed" version you found. (They are a simple steel rectangle with a "floating" center bar.)
The fact that you found them on a beach suggests they are strap-adjustment buckles from a lifejacket's cloth straps. Some of the geezers here, like me, had to wear that type of lifejacket when at the beach, and for canoeing, boating, etc. at Summer Camp back in the 1950/60s.
