Primarily because of the large numbers of them that were manufactured and are still in use, it's most likely a .38 Military & Police from the 1915 to 1942 period, but a bit more information would allow confirmation of that, such as: Is it in fact a Smith & Wesson as marked on the left side of the barrel? What additional markings are on the frame? Is it a 5 or 6-shot? Fixed or adjustable sights? Best of all would be a clear photo of both sides of the gun. If it is what I think it is, any markings inside the yoke cutout won't be of much help.