28D7GT/G

This is a mystery tube referred to by the service spec CV946. Probably a British octal variant of the US loctal 28D7. No trace of an octal base version found in Sibley, in the catalogs of the most known manufacturers and even in the AVO tube data. No idea if any production of this tube was launched.
An octal functional equivalent may be the 26A7GT/CV3577.

Some receiver tubes with Octal-bases are designated either with no suffix or with the suffixes "G" or "GT". Tubes labelled “GT/G” are actually type “GT”.

The suffix “GT/G” on the tube (or schematic) is intended to indicate that the tube also serves as a replacement for the older and larger type “G” tubes (“GT” type tubes can always replace the same “G” types).

There are no significant electrical differences between any of the glass tube suffixes, or with metal tubes (Metal tubes have a built-in shield, of course).

Since “GT” and “G” are electrically equivalent, we have blocked “GT/G” labelled tubes for use in devices (Otherwise, we would need to include all other equivalent tube types on the model page, such as 12AX7/ECC83), and because the same tube would otherwise appear twice (in statistics, etc.). See details in this forum thread.