The earliest reference I can find in my Motorola paperwork is for a P-9303 Single Tone Oscillator & Decoder schematic, dated 6/12/59.
However, these are not PL tones, looks like tone-burst instead, listed freq.
range is 600-3200 Hz.
BTW, tone burst is used a lot on places like Europe on VHF/UHF FM repeaters.
1750 Hz is the common freq for that.
--John WB0EQ
