It's not a very sought after item, although they may gain some popularity as the lightweights keep coming out of the garages and attics.More and more sense to me that speedometer.I used to throw a spotlight talk,now I've learned to save everything.A small fitting attached to a spoke and the odo attached to the fork.Each revolution of the wheel clicked the toothed ring on the odo once.You bought the one that was for your size of wheel.
They were known as cyclometers. You bought the one for the size of wheel, certainly available for 27", 26", and 20", and they were accurate to about 1%. They were available in km and mile versions and measured down to 1/10 mile. If you wanted average speed you needed a stopwatch and a bit of subtraction and division.
There were a number of different makes but the principle was always the same. The pin striking the wheel on the meter eventually wore both away. I recall having to buy a new one about every 15,000 miles. I once worked out how many clicks of the pin on the wheel that was - lots.
Although the extra functions on a cycle computer are useful the cyclometer did its job well, without batteries, and very reliably.