That is a weird stamping for sure. I've never seen it. I know a few guys who shoot .410 shotshells (2 1/2") out of their .45-70s without issue. But accuracy sucks. The rifle you describe would probably be just the opposite being a smoothbore. I'd say you more than likely could shoot .45-70 loads out of it provided they are the low pressure loads for trapdoor type rifles. But to be safe I would definately get an answer from an expert. I didn't find anything on the net for .45-70 shot but there are some articles on loading shot shells for a 45-70.