Odd — doesn’t look like it would be feasible to mount it in a transom… if positioned low, you couldn’t turn the handle and it would be in the way of occupant traffic. The yoke looks like the metal is too thin for a heavy duty application.
My vote would be for some sort of vat or barrel where it is mounted vertically.
If the drain was on the hull bottom you wouldn’t have to bend down to pull the plug ..Lol!
Maybe used on an inboard wooden boat where access is limited ..