[GreenKeys] Wobbly FSK ?

[Nick England]

I recently added some info to my site on the KY-30/GRT, an early Navy
FSK keyer designed to take RF from a transmitter's oscillator, mix it
with an FSK signal, and send it back to the transmitter for
amplification.
http://www.navy-radio.com/rtty/ky30grt.htm

A description of the TTY input keying voltage which controls the
keyer's local oscillator says "It may also be wobbled at about 200
cycles per second by turning on the PHASE MODULATION switch. The
amount of phase modulation is varied by adjusting an associated dial.
Phase modulation of the transmission may tend to improve reception
under some conditions of severe selective fading."

Was this wobbly phase modulation a common thing back in the day?

cheers,
Nick K4NYW
www.navy-radio.com