O a Birmingham proofed gun of that era there should be a gauge mark for the more of the gun, no diamond or other enclosure & then the Chamber mark. This will be either 12C or 12LC (chamber & long chamber) It is debatable whether the LC includes 3" chambers or only picks up on chambers longer than 3". It seems 3" chambers have been found marked each way. If it does not have the Nitro mark then as stated it is black powder proof only.
For a number of years, British guns with smokeless proof were actually in fact proofed with a special Black Powder proof powder. This powder gave a high enough pressure to make the proof suitable for smokeless.
I am not familiar with a British proof mark of the gauge in a diamond without either the C or LC
