Vintage Brauer Bros. H37 leather belt holster.  Brauer Bros. catalog lists this for 4" large N frame Smith & Wesson revolver such as model 20,21,22,23,24,25,26,27,28,29,Triplelock,1917,1926,1950,etc.  Should also fit large frame Colt revolver and similar.  Pictured is a S&W K frame 19 which also fits quite well.  Excellent usable condition as pictured.