H Williamson WWI - how to remove case back?

I have an H Williamson WWI watch passed down through the family with a fairly good condition case - only minor scratch on glass. Unfortunately I cannot either wind it nor adjust the hands – to an engineer (but possibly not a horologist!) it feels like the gears have stripped. The front glass is easily unscrewed but I cannot remove the case back which I would like to do before considering taking it to a specialist watch repairer – how is the rear removed?

If there is no "notch" for a case knife I would think it is a screw type. Try turning the back counter-clockwise.

Actually that looks exactly like a swing out case I have an old Hamilton in. To swing the movement out you need to put the crown in the setting position.

While there was a discernible line around case back I could not see a knife notch and did not think a military watch would be sophisticated enough to have a swing out case so just continued to try and unscrew. Eventually I did it and found the reason see photo! I suspect one of my previous family had a repair attempted only to give up. Interestingly, well at least to me, is that the case was made in the US - The Illinois Case Co. Thanks for the help - this is a good site.