Dyed and drilled nylon. Three versions, each with 2 screws, series of 10 each:

1. External Diameter = 72mm, internal diameter = 40mm, depth = 6mm.
2. External Diameter = 60mm, internal diameter = 34mm, depth = 5mm. 
3. External Diameter = 50mm, internal diameter = 29mm, depth = 4mm. 

Diagram Nylon Marker II

Quay Hill, Falmouth UK (72mm Diameter)

Lumeijstraat, Amsterdam, Netherlands

Quay Hill, Falmouth UK (50mm Diameter)