Welcome to the forum and the wonderful world of M20s...It fits to the steel 'cap' that covers the upper end of the oil filter...When the oil tank lid is closed the underneath of the lid exerts pressure on the spring, keeping the cap in place on the filter....Ian

email (option): ian@wright52.plus.com

There must be two different types, the one on mine fits over the filter.

Here's a picture of a replica assembly..It looks quite like the original and illustrates the complete fitting....Ian

email (option): ian@wright52.plus.com

My first bike was an M21 back in'71 when I was still at Boarding school. I later got another at College in 75. They both cost £30.
It wasn't till I got a third one 10 years ago that I discovered this part, and then only whilst browsing Draganfly parts files...
I've belatedly come to the realisation that I could have saved myself a great deal of time and money if I'd always had one. The second I had to let go after being chased by a police minivan in Mid Wales. It's spark was so weak I often had to close the gap right down and then at the top of a hill open it again. I used to start it by spinning the engine up and letting out the decompressor. On the motorway I drafted coaches. It was a typically worn 70s bike. Most of us were getting round on 50's relics. Anything with girders was looked upon as being out of the Ark.....

Thanks! Makes sense.
More questions will follow probably ;)

email (option): info@erfgoed40-45.nl