It is just what you mentioned in your post. I rebuilt my 350x rear caliper and it has the same small cupped washer. It holds the o-ring in the caliper hole to seal the rod coming out of the back of the piston. So the o-ring goes in first, then the cupped washer. I found one of my small 1/4" drive sockets that fit perfectly into the cup and use that to lightly tap the washer into place. Don't it hard or else you'll just flatten the o-ring.
- Frank
1984 200ES Big Red
1985 350X (x2)
1986 350X
1986 250SX
1984 Auto-X
1984 ATC70
1985 ATC70