OK, Unfortunately, I don't have any reset knobs left. I've got a picture, and I do have all of the dimensional data. This was for an earlier style speedo. The later style is a different lenght. I don't remember if it was longer or shorter, but I can call someone I gave one to. He has a TR3B, and I think he said it stuck down farther then his original, which would mean that the later one is shorter. I'll verify that.
Anyway, here is the dimensional data.
Shaft -- Diam. 7/32" (0.218") Length 1 19/32" (1.593") not critical.
Drill a 1/8" (0.125") hole in top of shaft 1/2" (0.500") deep.
Cross drill for cotter pin. Center of hole 1/8" (0.125") from top of shaft. Use a #52 (0.0635") drill, and drill all the way through. Then use a #45 (0.082") drill and drill through the same hole, but only go through to the hole drilled earlier in the end of the shaft. This will give you two different size holes where the cotter pin goes through. The reason for this is that the end loop of the cotter pin has to go partway into the shaft, or it will not clear the opening in the speedo housing when pushing up to reset the trip odo. Also, the ends of the cotter pin have to wrap tightly around the shaft, so cut the length if neccessary.
The other end has a brass knurled knob. It is 3/8" (0.375") in diameter, and 5/16" (0.312") in height. It is pressed onto the shaft, so drill for an interference fit. I knurled the brass piece after pressing on.
Hope this helps.
Sorry I don't have any left.
Edit: The one in the picture is an original that I used for dimensional data.