RFC was probably one of the first Datong products for the amateur market. It could be purchased either as a completed unit or as a PCB module  RFC/M. 

The principle has also been used in commercial equipment - I have found a similar circuit used in Park Air 1400 and 1250 airband transmitters.

• Download RFC/M manual and circuits PDF (0.99mb)
• Download alignment instructions PDF (112kb). 

 Finally, thanks the the late Jan-Martin LA8AK for tracing out the above circuits.