To give you an answer with any meaning, we need model, finish and condition. You question right now is sort of like "I have a Ford V-8 truck- what is it worth?" Good questions= good answers- please repost with some information, and we'll do our best.
WHERE did you find 'usmr' ////////////////
usuall on the barrel will be encribed the maker. If colt, it will indicate Colt ptd firearms, etc, etc
Probably Uberti. Look on the underside of the barrel for manufacturer's logo/markings. The "USMR" marking is historically accurate and appeared on original Walker revolvers. Stands for 'United States Mounted Rifles' which was the regiment of the designer. sales@countrygunsmith.net
barrel or frame
I show these made from 1980-1982. In any event, mint condition guns will bring $300-$350. sales@countrygunsmith.net
Italian most likely
look for the country of orgin.all guns should say made in[ usa,Italy,Spain ]If it says USA and colt,you can contact the Colt Historical socity and they can provide you with the infromation. If the country of orgin is any other country,there should also be an importer stamp on the gun somewhere [ Navy Arms,amri san Marco etc ] and youcan contact the importer. Walker Colts have been reproduced several time in the last 40 years.You mayalso contact Gun Digest,Guns and Ammo or SASS [single action shooting socity} a group of western or cowboy enthustist Good luck Bear