I found a site about the Peace Symbol of the fifties:
Peace Sign - The Peace Action Symbol was
designed on February 21, 1958 for use in the first
Aldermaston Easter Peace Walk in England. The
symbol is the composite semaphore signal for the
letters 'N' and 'D' standing for Nuclear Disarmament.

The semaphore symbol for "N" has both flags lowered to forty five degrees.
The semaphore symbol for "D" has one flag overhead, the other at the feet of the signalman.

To me the joke was that the circle with vertical bar, and the symmetrical slanting bars
looked very much like a huge bomber aircraft.