Maybe you are looking at the wrong holes in your engine plates? The stud is 3/8" and also holds the rear of the mag platform. The smaller hole behind it doesn't seem to have a function? But knowing Matchless, it's probably one of a series of ten different engine plates.
Ron
email (option): ronpier@talk21.com
Just whip that dynamo off Ron would you?...
...Ian
email (option): ian@wright52.plus.com
...Ian Doesn't the current forum picture show a small team engaged in the preparatory work to remove the dynamo from a G3L ?
You're welcome Richard. I've been there, with lots of pictures except the very one you want is just out of sight
We joke about that dynamo idea. But it's no joke really! I would make sure that the dynamo is professionally restored and that new chains are fitted and lined up and adjusted correctly. I loaned a G3L to a friend some years ago as his own Triumph was in pieces. The bike had recently come to me and I hadn't yet touched it. He managed to do the event in Guernsey with it, but continued to ride it afterwards. Of course the chains clattered together and jambed up in the primary case. So he had to remove the case by the road side and throw the dynamo chain away. I received the bike back with a bent armature 
It's not my favourite design idea. Ron
email (option): ronpier@talk21.com
Not if its bent there isn't. All you can do is source a secondhand one. As its now scrap you could try wacking it with a hammer between a couple of bricks, you never know it might work!