The tubes for the Faema machines are standard tubes, but the electrode caps on the ends were custom. The end caps then snap into the clips inside to make the electrical connection. So the overall length is the same as the distance between the clips.
I did actually make some tubes and also planned on selling a kit, but the things run on high voltage from a neon transformer and I thought better of it. You can see the replica tubes here:https://www.instagram.com/p/BTcveq2DQFz/
They are great for my own machines but I'd want someone to sign a waiver before selling them
If you want the specs I can measure the tubes.