The attachment you are describing was readily available commercially in
the sixties and seventies when it was known as a powerhead, and used on
spearguns, hand spears and hawaiin slings to slaughter sharks, groupers,
rays, etc. I remember the joys of exploding jellyfish. Usually the
powerhead was carried on the waist belt, and then clipped onto the spear
over the usual head when needed.

The authorities initially ignored powerheads, or were unaware of their
existance, and then decided that they were concealable weapons with all
the licensing requirements thereof.

If you cruise internationally firearms of any form can be a major head
ache, and concealable firearms even more so.

Fair winds

Graeme