Thank you both for your pointers, however my problem is that the picture clearly illustrates a boat underway with an early type conning tower and there is no deck gun. None of the boats that are known to have used this heraldic device were of a Type VII/A design, as far as I am aware and all pictures of the emblem I can see in the various reference materials I have, show it sitting in line with (and hence underneath) the support railing of the upper half of the conning tower, which my picture does not show.

The search continues!