The circles were not standard issue, but one of the suggested ways to improve your radar "experience":
Quote:
The following suggestions will speed the obtaining of data from the PPI and increase the accuracy: 1. Add inked circles on the face of the PH tube for estimating range (four solid circles, interspaced by four dotted circles). Care must he taken not to scratch the tube in any way.
2. Improvise a more accurate 360 degrees bearing circle over which rides a cursor, or thread stretched across the screen. Targets may then he split by this thread and hearing read on the circle.
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Taken from:
http://www.hnsa.org/doc/radar/part4.htm#pgSJ-1