They are small two speed transmissions, with the same % difference in ratios as the rear axle has.
If not mounted directly behind the gauge they can be anywhere in the speedometer cable, second most common place is on the firewall.
If behind the dash it's likely mechanically shifted.
If you ave an electric two speed then the adapter could be electrically shifted. Not sure if they ever shifted the adapter with vacuum, but probably?
Grigg
