On the road-going Comet & Meteor, this part is known by the number F117.
Basically, it is a sleeve that enables the stud, that passes through the frame tie, to be extended. This allows clear access to the nut, allowing it to be easily removed/replaced, and the spindle withdrawn when the bike needs to be stripped down. This part is unique to the Grey Flash and would probably have been allocated the part number F117/1.
We were glad to learn that an original example was present and would have been the part the bike left The Works with. The fact that 2751 features so many more original components is indeed fortunate.