Nice view of the dolly. The wheels appear to be the most common type, with 8 bolts holding the two wheel halves toegther. The purpose of the hole escapes me however; possible it gives access to the tyre valve. The small piece of wood hanging from a cable under the dolly beam is rarely seen; the only other example I know is this picture of the Duxford Komet. It is probably attached to the cable seen in this picture of the Science Museum example. My guess is that the piece of wood is inserted in the dolly release mechanism mounted on the skid, and it being pulled out signals that the dolly has actually released, and the skid can be retracted.
Photograph by Dave Brooks, August 1999