[Milsurplus] Markings on WWII Navy Receiver

[David Stinson]

I recently acquired a WWII U.S. Navy DAG-1 portable direction
finder receiver.  It has been marked with a black grease pencil.
On the front cover is "ICP / SUR / 13-64."
Before that was written, someone marked it "DAG" with
a white grease pencil.
On the sides is written "ICP"
On the back is written "SUR ICP 13-64."

Does anyone know the significance of these?
I'm trying to decide if they should be preserved
or covered with a repaint.

Thanks,
Dave S.