I'm guessing that example is late 1920s.
When tubes were EXPENSIVE and low-performance, best results were got with a "general purpose" (what we'd now call a fairly low-
Mu) triode and a step-UP transformer to the next grid.
It was aMAZing to hear sounds from the air. Listening was either telephone earpieces or a telephone earpiece coupled to a horn. 300-3KC response would be generous. More important was GAIN. Your part is speced for a 1:5 ratio, a stunning step-up from the relatively high impedance of a tube plate.
I don't see what you want it for except as a paperweight. There's many other ways to get a narrow-band response for special studio effects. R-C filters. Digital filters. Transistor radio transformers: Mouser has most of them.
It is a lovely object, I'd offer $2 just to look at it a while.
Plate-to-grid transformers ARE very useful to radio restorers. I dunno how valuable they are today. It seems like everybody and his sister was winding transformers in the shed, also catalog suppliers. I know I have seen them offered inexpensively; also it is possible (not easy!) to rewind an original transformer which has failed.
Here you have Terry who actually knows this obscure radio maker and would surely give it a good home. I think the universe is giving you a hint. Get his contact, show it to all your firends, check around, and then do a deal with Terry.