5 inch/ 51 caliber Naval Gun it is!
Looks like I stumped the band again!
Gun is from the Secondary Battery of the Battleship USS ARKANSAS BB-33, photo taken in 1919.
Blooz, get the points - device is a .50 cal targeting rifle. It was used to aline the gun, by tracer, when targets had got so close that the the telescopic sites were ineffective.
Smids, sorry, both ships in the background ARE US Destroyers. Don't think any RN DD's ever had 4 stacks. Good guess on the timeline though!
Congrats to all!
