Need help with speedometer/odometer

[FordTudor1931]

It takes a lot to move all six wheels at the same time, especially if the grease is a touch old and stiff. Since the rotation of the speedometer cable doesn't stop, something has to give. It could either strip the cogs or move it back out of gear. The back popped out and the gear released. I don't know this for a fact, but it could have been designed this way to avoid breaking the drive and allow for a fairly simple repair.