When I was a kid there were a few around Indiana. It wasn't enforced and soon went by the way side. Look out with many states trying to increase revenue we may begin to see it Image Gallery
The plate may be a thing of the past... I see some tractors with it - especially commercial plates and some farm tractors without. "They" say if you are on the road a lot you should have one. I was fortunate enough to transfer my old "non-expire" farm plate from my previous tractor - one time fee and registration. Now if you get one you have to pay an annual fee like a car or truck.
On my JD 3320 TLB there is one SMV sign permanently mounted on the back of the seat. If the seat is all the way back there is a bit of contact between the BH seat when in the "use" position and the sign.
In addition, there is a second SMV mounted on the BH bucket cylinder with a little bracket that allows removal of the sign, but the bracket stays on the cylinder.