I did comment that you need to specify the country or region of that time; Police uniforms vary in style throughout the world now and in the 1920s.  It may not make a difference if your rendered depiction is not part of a storyline or an exact historical scene but if your period scene is reproducing events of that time for a specific location, you would want to build your stage as close to accurate as possible.  That's why most of the suggestions are rather ludicrous.