Hi mac73,
all the bikes have only one washer placed in between the spring (point "A" of the picture from trialtrial) and the selector drum.
I had to replace the selector drum+selector shaft last december and I had the same problem than you!
I was sure that one washer was between the spring and de drum but I didn't remember the other washer on top of the drum. But when I received the parts from my dealer, a washer was already on top of the drum. So I decided to assemble the shifter shaft with two washers, one in each side of the drum...
After wasting my time for a complete weekend trying to put back the gearbox (but having the same problem than you) I decided to ask my dealer...: the factory ships the shifter shaft + drum + SPARE washer on TOP of the drum, but it is only because it is easyer to ship the washer on top than in the lower part of the shifter shaft!