[Arran]

The reason why 6T5 tubes are rare is because brand Z only used them for a year or so, I'm guessing that they were made for them by Sylvania. I'm not sure how long they were available as replacements but they were basically an obsolete tube the year after they came out with them, probably a question of economy of scale. The difference between a 6T5 and a 6U5, 6G5, or 6E5 is that not only is that center cap missing but it has no deflection vanes for a shadow like a normal eye tube would.
Regards
Arran