I'm gonna walk back my statement in the earlier post. I got into looking a little harder, and the shell pictured in Dickey \ George has raised bearing surfaces, while yours has recessed with probably lead or copper bearing surfaces. But, both shells have that same flame groove cast into it. Very similar. There is also a shell called a 'Mann" shell I saw somewhere that resembled it. So, my opinion is not valid, and I really do not know what it is. There is always the possibility of experimental CW period, or post war which has so many shells that there is not much knowledge available about.
Come on to the show in Richmond this weekend of the 16th and 17th, Carl, Mike , Pete George and myself will all be set up there and we can all take a look at it. Then we can give you 4 different opinions !!