Hello Mike,
I've lost patience trying to download picture and sketch here, look in my photos for Tom Senior Drive pin, I have sketched the dimensions from my 'Junior' and taken a photo of how they are arranged, pin to spring, on the machine.
Release by pushing the spring away to engagethe pin in the two postions
I hope that helps you. the pin is soft by the way, easy to make and the spring clip would not be difficult to make up either
Good luck Brian