https://www.shotgunworld.com/threads/w-c-scott-son-help-with-id-and-value.242236/
In 1883, William Middleditch Scott patented a self-opening action (No. 727) for back-action locks (made between 1882 and 1887)

If indeed pre-1887 the original proof marks may have been on the barrel rather than the flats.