I'm making this post in case any G3L owners are not aware.
Some years ago, quite a lot of pannier frames appeared on the market with slider sides as opposed to wing nuts. I did a lot of searching through pictures and came to the conclusion that these were fitted to Matchless G3L's. Probably produced by, or for Matchless as an economy measure.
I have seen one picture with them fitted to a Norton 16H, but it never left the factory like that. Here are a few pictures from my collection.