I am very saddened that your prototype tube has cracked and gone to air, especially the fact the tube was finally up and running displaying good colour pictures
, if something like this happened to me with my projects I would be weeping buckets!
Knowing RACS having successfully rebuilt a 15GP22 and there being possibility they can do the same with your prototype tube gives some ray of hope and I do hope after your discussion with Filipe it will be agreed that it will be feasible to try getting the prototype tube rebuilt. Anyhow despite the sad news, it was really good that you brought this tube back to life while it was in working condition and got it to display excellent pictures! Anyhow best of luck with whatever possibilities there is to get the tube rebuilt!