[GreenKeys] oddball mark space frequencies

[David Burns]

A Google search reveals it is a keyer unit, part of an AN/TRC-90 etc. 
set "Voice-Frequency Telegraph Equipment".  The frequencies correspond 
to Channel 7 (out of 16).  Shades of the 1960's.

http://books.google.com/books?id=MdYXAAAAYAAJ&pg=SA1-PA2&lpg=SA1-PA2&dq=frequency+2337.5&source=bl&ots=cZmqn2fEVy&sig=00lOTxHlrqdNgtZew48IFLRnvd0&hl=en&sa=X&ei=x2svUfipFIiK0QH17YHYDg&sqi=2&ved=0CFIQ6AEwCA#v=onepage&q=frequency%202337.5&f=false