Here is another look.
I dont think that's 5 rather it looks like capital E. "E" for excellence. Navy ships usually have these on the bridge wings.Under it appears to be stripes for the number of times they achieve it.
This is a guess but perhaps the captain wanted to create a spirit of competition among the gunners.

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These photographs show USS Mugford DD-389 and USS Selfridge DD-357 moored at the PH Naval Yard on September 9, 1941. One thing that caught my eye is the number "5" painted on the side of Selfridge's #3 turret.
Anybody have any idea why there is a "5" painted on that turret?