Military and civilian barrels were about the same in thickness, thin by today's standards. The butt markings are indeed indicitive of British service, but not necessarily as we see the gun now. It's very short compared to the regulation 46-42 inch barrel besses, but officers and senior NCOs might have shorter arms. Alternately it could be a recycled military arm either cut down and reduced, or a militia cobble job, American made from used parts as mentioned above. Hard to tell, but in it's current state it's would most likely denote either private purchase or militia arsenal inventory. Neat piece.