Could 'Tapered' rimbase refer to the taper against the tube? The 18 pounder (one of few tubes shown) in Nelson has rimbases as the tube has a slight taper; the two Howitzers shown have straight tubes from the trunnions aft, and taper forward to the muzzle (they don't appear to have rimbases).

My 7-1/2-inch cast-bronze American Napoleon tube has rimbases, at the point on the tube where the muzzle-taper transitions back to a slight taper to the breech; the rimbases are like those on the 18 pounder.

Regards, John