There are two sizes of gaskets required for the 3+1/4inch Pre-WW1 Davison sight glass. A 31mm OD gasket to fit in the top cap and a 29mm OD gasket to fit on the base of the glass. One Top gasket and one Base gasket should be installed.

80mm diameter “Lucas Cadet” type Convex Carbide lamp lens.
With polished edges.

Available in black or red - price per foot

1+3/8-inch X 1+5/8- inch Crushable Copper gaskets with asbestos substitute material inside. The typical Imperial size for your Douglas motorcycle valve port gaskets

Douglas Flat Tank- Oil sight glass and gasket kit. The kit comprises a 39mm diameter ,2mm thick, hardened sight glass plus a top and a base gasket. In addition, there is also a fine metal shim in the kit which is placed below the base gasket

Petrol/Oil cap gaskets, 2mm thick (for all- metal fuel/oil caps)
Suitable for many types of flat tank motorcycles.
O/D 1-7/8"
I/D 1-1/2"

Part Nr 66-462 Triumph oil sight glass and gasket kit The engine mounted sight glass and gaskets for the late 1920s early 1930s Triumph. Models TT, ST, W, WS, N & NSD. This sight glass is best suited to the type of sight glass holder with the Screw-in type of retaining ring for the glass and gaskets and not the Push-fit type of retaining ring