Ed,
Correct, I was just about to start typing a reply.
Brad,
I conducted a google image search and first came up with this image:
http://www.navweaps.com/Weapons/WNBR...er_m8_diag.jpg
It then lead me to this site:
http://www.navweaps.com/Weapons/WNBR_2pounder_m8.php
The date of 1957 on the sights don't seem to fit with the description a weapon designed in the 1920s but potentially some were still in service (or the mount was adapted for a different weapon).
Regards,
Colin