What is this?

[Thrift Store Scott]

Speedy wrote:
Any Model T owners here? I think Paul F is on target. It appears that one end goes into the axle spindle center and the other wedges between a tire and rim. The hook can grab a spoke. Why you would need to do that, I can't guess.

Absent a better explanation, that one seems the most logical. If I recall correctly, some Edwardian cars with spoke-type wheels had the tires mounted to metal bands or hoops which then were clamped to the wood and metal wheels. Here is a pic of a 1911 Thomas Flyer in which you can see both the spare tires on their bands and a decent view of the clamps on the front wheel.

I'd guess the hook attaches on to the leaf spring and tightening the nut forces the band/tire assembly to go into place on the rim while it was still on the car.
If this is the case, it would certainly help explain why (easily) de-mountable one-piece wheels were considered such an improvement when they were introduced.