Black rubber seal from Kenwood Chef

I have just replaced the vertical drive shaft in my Kenwood Chef KM0101. At some point, while opening the gearbox, a black rubber seal fell out and I can’t see where it goes. It’s rubber so not a washer. It is slightly tapered in external diameter from one face to the other and has a groove in the wider face. If anyone can tell where it goes, I would appreciate it.