It is a .38/44 Heavy Duty. SN 52501 would date its shipment to around mid-1937. At least I have listed four other HDs having nearby SNs which did. The finish does not seem original, and the grips on it are definitely not, they are from post-1969. The .38-44 (not .38/44) cartridge is simply a more heavily loaded .38 S&W Special, MV around 1100 ft/sec. It has not been loaded by the major ammo factories since the early 1970s. You should definitely check to see if the chambers have been lengthened to accept the .357 Magnum.