Hello Dee, the exact figures may be published somewhere, perhaps someone will know
In the tech manual is a registry of members vehicles, past and present. Last time i counted (sad i know!) there were roughly 800 cars registered, and out of the 800, where the 5 speed gearbox option was listed against their car, these are the numbers:
230's - 6 cars
250's - 6 cars
280's - 7 cars
It's only a very, very rough approximation, based on the details the owners provided for their cars, and some of these may have had 5 speed's retrofitted, but it shows that less then 2.5% of cars had the 5 speed box