2 3/4 exhaust valve cap question

[gsx1100s]

Hello , I hope this finds you all well.
I have a question concerning the 2 3/4 engine. I have a 1918 motor. Instead of a normal  exhaust valve cap  , I have a similar bolt .with what appears to be a conical finning on top , it is about 1.5 inches long and has nine discs on it. These discs are slightly smaller than the previous disc under it , thus the conical shape. On the base of the bolt are printed the words " To fit Douglas M.G.C."
Many thanks in advance ,
Michael

[Doug]

Michael,

Fir cone caps. Aftermarket accessory that was supposed to help the engine run cooler by providing more radiating surface. I have never seen then on a Douglas, but certainly have seen them fitted to other makes of the teens and twenties.

-Doug

[gsx1100s]

As always , thank you for sharing your knowledge

cheers michael

P.S. after receiving your information I found a picture of them on a JAP motor at this address :

http://www.cybermotorcycle.com/gallery/classics_s/Stylson_1929_250cc_JAP.htm

[Frank Lyn]

A set were on a 1919 2-3/4 that I owned very briefly,   
What caught my eye was that the front one was shortened to clear the front mudguard. 
The bike never ran while in my possession so I cannot comment upon their effectiveness