Unusual Sulky Design

I just went and picked up a find last night - a new to me sulky. Found it on Craigslist. The attached photos were from the add, and I'll post new ones as soon as I get a chance to take them. I had to go get this after work last night, and it was a long way off. I didn't make it home until dark, so it's still in the back of the truck. The steering is frozen up, but I'm guessing I can get it fixed. For $25 I couldn't resist something this unusual.

I've never seen a sulky like this before, and hope to use it on my Gravely 5265 Commercial. I haven't measured the mounting fork, but it looks like it will work, or that I can modify it to make it work. I doubt that this is a factory Gravely attachment. I did see one foot steer sulky at the Gravely mow-in last year, but it was modified from a conventional steering sulky. Any information anyone can provide on this unit is welcome.

A few interesting design notes:
1. The leaf spring is mounted using 4 bolts passing beside the tube, but the spring is still drilled with two centered mounting holes as a standard Gravely spring.

2. The tires are Goodyear, 4.8-8, but I've never seen this tread pattern before. It's larger than the standard "Stud" pattern that they used to use on the Gravelys. One of them is completely shot, so I'll probably have to replace them. Does anyone know if this pattern is still available from someone else?

3. The rims are a heavy split rim design, so new tire installation should be fairly easy. These are not typical Gravely split rims, as they have a center hub

Please let me know any information you may know about this item. I love to study old equipment. I hope to restore this sometime mid-summer, after I finish my roller rebuild.