[R-390] letter on R-390A front panel - what might it mean?

[Phil]

My Collins R-390A [1955 run] has some markings on the
front panel - looks like permanent magic marker - near
the KC knob.

Being a rather historical person, as well as an
electronics nut, tube-o-phile and MW DXer, I'm as
curious as can be as to what they might mean:

There is a slightly styilized "F" which is circled by
a shape very reminiscent of a 1960's era TV screen - a
rounded square, if there is such a thing.  Also, near
this "F" are the letters "AOD".

Any guess as to what this might mean?  I figure that
perhaps some you you kind folks who worked with these
babies in the military might know.

One thing for sure - my front panel is in very nice
original shape and those letters are staying!

Thanks!

Phil Rafuse
PEI Canada

PS - I bought the R-390A from an ebay guy in upstate
NY.  R-390A was missing quite a few parts, including
the PTO, oldham coupler etc.

Phil Rafuse 
PEI Canada

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