In those days nobody even knew what the concept of ecology was and the motto of those who went underwater armed with a speargun was simple: shoot everything worthwhile to bring the richest game bag ashore.
It was 1948, mask and fins were still children, fish was the goal of diving for 99% of the few divers of those times. Losing a wounded prey trying to escape would have been unforgivable and so here is the trident bayonet with which to block any ambitions.
It was attached to the end of the barrel of the spring-loaded speargun, ready to strike, as the image above taken from the spanish Nemrod catalog perfectly clarifies.
The caption clarified that it was also used for defense, from whom or what was not specified.