Ok found them,, the M10 nut and bolt in my pic (sorry can measure the pitch) is not the same as the new yam nuts, its screws on 1/2 turn and locks....its a slightly finer thread hence I could not find any new nuts to suit and had to order them...you can visually see its slightly different.etc
These are the part numbers for your 85hp, 90185-10051
NOW im assuming your thread is the same as the Yam 90 two strokes and the same as my 2004 60 t/s which is what I have here.
A pure guess that yours is the same thread..and they have just altered the part number 25 years down the line to the pt no i have shown.
By the way, you can have them for £100
I would send them up to you but i need em!