Thanks for the offer, but I think it's going to be a no. The Yam thrust washer is quite complex, with internal and external tapers, so giving you accurate dimensions is almost impossible without knowing what we need to achieve. From what Pete says, to fit a Merc prop on a Yam, it sounds like we would need a thinner thrust washer rather than an extra washer. And the oposite for a Yam prop on a Merc.
I rang Fairweather Marine today and the chap I spoke to reckons that the Mercury and Yamaha output shafts are different dimensions
and that they don't do any adapter washers. He suggested that a Laser II with a Flo-Torq hub could be re-hubbed to fit the appropriate engine type.