In July 1981 you could read about the 10MHz 68000 on usenet, in the fa.info-micro group:
"Designeers requiring a 68000 faster than the current 8-Mhz microprocessor will gain 25% in throughput with the 10mhz version. The fast microprocessor is the result of Motorola's program for yield enhancement."
The announcement came from a magazine called Electronics from June 16, 1981. News travelling at ... well, at most 1200 baud, I guess.