Cleaning under and around my air filter housing and this rolled out. 
can anyone tell me what it is/does?

it’s like a hard rubber screw in stopper.

Number 15 on this diagram is labeled as an over flow valve. It could be said item. It would be in the general area that I found it.

If it is indeed the overflow valve, what does it overflow?

Now I know it’s not #15

 

If you look at the bonnet a couple of these are usually screwed into it at the front, one on each corner. Not entirely sure it’s purpose but it adjusts the height of the bonnet when shut and acts like a cushion of sorts I expect. The screw thread at one end allows it to be screwed into the holes and height adjusted 

Legend. Last place I would have looked.

thanks

Edited 8 November, 20205 yr by Rkdee