If you look at my wheel list, I have not actually given a date when the Mk6 wheel design was in use, certainly it's first usage on a Corgi branded model, so I have no problem with your earlier dates for this wheel. I just know the Mk6 design has been used on 2003-04 Routemaster castings...
I can't imagine many were made. I would have thought it's a gift for potential customers.
The link below is to a newspaper article from 1993. Sad reading, especially looking back at what London Transport's Chiswick and Aldenham works must have been like in their heyday. Just look up "Aldenham works"on Youtube to see some videos showing this amazing facility, which could overhaul 50 buses per week.