[Jake Blake]

" They chose to use Sylvania-built ST glass tubes with octal bases instead."

Thank you Ron. 
This actually may answer another question I had, about which tubes were original in my Motorola 51x17.  I originally guessed the one Sylvania it has is a replacement. . . now it's looking like just the opposite... possibly it's the only original !  That gives me a clue to which one most likely just died.  I just eliminated the rectifier with a switch-out.

"To be certain look at the data sheets for the tube number with and without the G or GT."

Thanks Mike.   The schematic for that Motorola calls for all GTs.  Interesting.