Original full nitro (not reproved from black) underlever hammer guns are relatively common. Hammer guns were made in quantity much later than most think. Also, original nitro proof was available in the UK from 1887. If you like hammer guns, you need to learn the proof marks. Nitro re-proof from black is easy to spot. There will be two full sets of proof marks. It is dangerous to fire full nitro in a rifle only proved for black. The purpose of re-proof to nitro is to establish that it is safe in that particular rifle.

Champlins had a really nice William Evans underlever hammergun in .450/.400 Nitro for sale some years ago. That rifle was made in 1937.
---------------------------------------------------------------------------