Here is a close up of the front sprocket shaft seals with the Yamaha parts numbers. The wide one is the outer seal.
I have two "complete gasket sets" for TW's on the shelf and neither set had the shift shaft seal in it, or for that matter the drive shaft seals or the kick shaft seal. So much for being complete!
Yamaha part number for the shift shaft is: 93102-12321-00
Good luck. Tony