The ".56-52" marks indicate a civilian application. The military had no designation of ".56-52". It was either a #56 for the 1860's (.56-.56 ... the measurement of the exterior of the cartridge just ahead of the rim and at the mouth of the case), or .50 caliber (.56-50 ...again designating the nominal dimensions of the case). The .56-52 cartridge was a creation of Spencer himself, who felt the government cartridge had too much crimp on the bullet. So he bottlenecked the cartridge. Thing is, the .56-52 will fire interchangeably with the .56-50 round in the .50 caliber chambers. So if the gun is stamped ".56-52" it would indicate someone had a preference for the commercial version. That leaves an open question. You say the barrel is "original", yet has six lands and grooves, with a groove-groove dia. of .518 at the muzzle. That is puzzling. I think it would be worthwhile to make a chamber casting (using CerroSafe low-melting point bizmuth alloy...Dixie Gun Works or Brownell's sells it) to see if the chamber is straight or bottlenecked. If the latter, then I would say the gun was rebarreled a long time ago (probably just after the CW when these arms were being surplused out by the Army), possibly by an outfit like E.C. Meacham of St. Louis, or some individual gunsmith.