I am looking at a really interesting German gun, It is chambered in 16ga 2&3/4, I want to make sure its in original proof and has not been opened up here in the USA.

My question as a novice is, did the Germans use 16 2&3/4 shells/chambers or 2.5 like the English and if they did use 16ga, how would it be marked on the flats?

Thank you!