Jay-
Computers might be off topic, I forget when the Avocet was introduced. but there was a Speed and Cadence monitor made in 1974. the name escapes me at the moment, was made locally here in Southern California I think and not cheap, like $400 at the time, it used analog needle displays, and hall effect sensors, pretty advanced back then.
The readouts and processor box with batteries was about 1" thick, and about 3" top to bottom and 5 to 6 " wide.
Ron Skarin used one and also raced it to victory in the Senior Mens Pursuit Nationals, and the use of it was challenged prior, but was pushed aside.
John Jorgensen
Palos Verdes Ca USA