Float bowl nut seal for the Dellorto PHB and PHBE carburetors.
Fits the 17 mm (spanner width) varieties of that special nut:
The inner diameter is some 18 mm, the outer diameter some 23 mm and it is around 1 mm thick.
Required quantity per carburettor: 1
This is item number 40 in the drawing.
Dellorto Part Number: 4057