There are also two different mounting brackets. The later version (model 25 and 35 perhaps?) has much wider contact spacing and uses a conventional sensor and spoke magnet. I believe the model 25 is pretty dumbed-down for it's time though. I insist on a proper ride-timer that stops when I stop and that thus accumulates an accurate average speed, a feature not found on lesser-model Avocet computers.
But computers with too many functions like cadence and count-down timer are no fun to use out on the road.

Magnet rings can be removed from their mounting ring and glued to the hub flange with silicone RTV for a tidier look. A spacer ring can be something like a plastic cog spacer if needed, but most Avocet sensors have a lateral adjustment feature that is indexed.