On early Fords, the rear axles have a keyway cut into the tapered end of the axle where a steel key is fitted.
The wheels lock onto this key, and are then held onto the axle with a nut. 
Be sure to check your old keys, worn keys can damage the keyway and therefore the axle.

This is a 1928-48 Ford tight-fitting rear axle hub key that will prevent damage to the axle keyway.
This key is tapered 1/2" and the tapered end goes towards the differential.

• 2-3/4" Long
• 1/4" wide/thick
• 2 required per car
• Steel