Tom, the RST 2 3/4" shells are also relatively low pressure.

From your description and verification it's a 400 but lacks the deeply chiseled fences (and perhaps the wedges of engraving on the barrels as well?), I think you likely have a Grade 2. There might be someone else here who can confirm that. I can't, because unfortunately, the Scott book contains photos of only Grades 1 and 3. Grade 1 is essentially unengraved; Grade 3 profuse engraving of very fine small scroll, but with the features noted above. Sounds like yours falls between. Congratulations on a nice gun--at a really excellent price!