Hi BarryM
Thanks for the link, useful site if all else fails , also looks cheaper than another I found.
I have contacted Mark by mail to see if he has one lurking in his multi boxes of small stuff he has, never know..
The Units full name I have found is
Graupner Multispeed duo 500 .. Item Nr. 1979
And this is one of the 2 larger gears required for the gearing.