Mercury prop hardware question

[steplift20]

I just ordered a continuity washer that goes on my prop shaft, I used to have one and lost it years ago,I am going to put it back on but I'm not sure where it goes? I know it goes on the prop shaft and I think it goes on after the big brass fitting, if you know or have a picture that would be great and also ,what exactly does it do? What's it's purpose.