Definition:Avoirdupois/Stone/Linguistic Note

Linguistic Note on Stone
The plural of stone, in the context of weight avoirdupois, is not stones but stone.

When reporting a weight in stone and pounds, it is also customary not actually to say pound.

Hence prime.mover would (if asked) report his weight as "$16$ stone $6$" instead of saying "$16$ stones and $6$ pounds" in full.