I think it is for a shock absorber strap.
It is probably an accessory for a Model T Ford that, like most other accessories, is not needed for the proper performance, as designed, of a Model T Ford as was intended and as it originally rolled off the assembly line. As restorers of Model T Fords, our task should be to restore these marvelous vehicles to the condition in which they were originally assembled and rolled off the assembly line.
Tom,it is a neat find,although I wouldn't want to use it. If the top can be found,it would look good in the museum.
By were it's clamped, it could be for towing.
What do you mean what is it? It's a "Model T Ford Accessary Axle Clamp". It says so right at the top of the posting.