20" US military rack standard? Or?

[emrr]

I came across a WWII rack mount audio amp with Navy markings that at first appeared to be standard 19" width.  Turns out it is 20", but otherwise conforms to standard height dimensions.  I know of the WE 23" rack standard, but I can't find any mention of a 20" standard.  Anyone know the story? 

 

[jensenmann]

Some Siemens mounting frames for amplifier modules came in 20" width, too.

 

[emrr]

Thanks, that's useful. 

Someone must know the story on US Navy installs.