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.....