The washer is a very thin stainless steal, with locking tabs around the outside, the hole on the inside is made to match the spline's on the prop shaft. I don't have the boat anymore that had this Volvo
sail drive and I can't remember the model but without this washer, I can't imagine how the prop would be locked well enough without it. Hope this helps, if I remember correctly, the parts
diagram did not show the washer very well, and may be mistaking the plastic washer for the SS one. Like I said before, the plastic washer that was supplied with the prop nut did not get used on my boat because there wasn't enough room for it. I assumed it would be used with differnt prop combo's but in my case there wasn't enough prop shaft sticking out to use it.