.38 Largo. Casing is about a .38 Long Colt or even a .38 Special, HOWEVER- they were loaded with black powder or at black powder pressures. DO NOT fire modern .38 Specials even if they fit the cylinder. There were a bunch of these that came in from South America about 10 yrs ago.
behind the trigger gaurd it says spain the only other markings is on top of the barrel and it say american best cartridges are those that best fit the o.v. thats all the markings on it no maker mark or anything i had a gunsmith look at it a few years ago and told me it was worth 200 bucks but he bought guns from people for 100 bucks then sells for 600. he low balls people bad him say 200 probally means it more like 600 or 800 but im not sure my dad got a s&w 38. specal that was my great granddaddys its really old he carryed it as a constable in like the mid 40' late 50' something like that it still looks new but it looks just like the gun expect it has the s&w logo on it