I found in this site,on "BSA M type gearbox"
This section:
"15-270 clutch rod gland washer, felt, (later cork) 1937-1947 (3/16” ID x 1.00” OD x 3/16” thick, stretches over pushrod between ratchet assembly and inside of outer cover, supposed to prevent oil travelling along pushrod to clutch and also a secondary “seal” to prevent water ingress and oil leaks at clutch arm, felt acts as an oil lubrication wick and seal?, cork sort of works for a while, often missing, possibly replace with oil proof closed-cell neoprene foam?)"
As to Rear Mudguard Fibre Washer-
If you have a P.N I can try to locate..
Is the same as OUP front mudguard fiber washer EB242 (Page 11 on PDF)
And Oil level plug washer and oil drain plug (If I understand correctly-page 13)
And petrol tank washer on page 13.
So if this is correct, it should give an idea of the shape and measures of the washer.
Possibly can be obtained from “Triumph Norton and Royal Enfield Motorcycle Parts by Burton Bike Bits”