This valve was in a job lot of valves I picked up recently. It looks like a cross between a PX4 and a PX25, but with (apparently) a 7.5 volt, 1.2 amp heater, so probably closer to a PX25 in characteristics. Pictures below show the valve and its box, which is actually peppered with woodworm holes to one of its sides - this valve has been in storage, possibly in a damp old shed for many decades. Note that like some similar valves, the actual number as printed on the valve itself is T.V.250 and not TV250.
Even though this particular valve with a 7 volt heater rings a bell with me from somewhere, I can find no reference to it anywhere on the internet, other than a mention of it on Radiomuseum (link below) with a generic picture of a four pin B4 base - nothing about the actual valve other than filament data. The valve looks to be new and never used and in its original box and it truly is a thing of beauty (as are all these large 'globe' type valves, I've got other similar types), and it would be interesting to knock up a single ended amplifier where it would not only look quite attractive, but no doubt work quite well.
I was sure I remember this valve being mentioned somewhere, but can find no reference to it now, so this thread could turn out to be unique in being the only 'on-line' reference to this valve type number. I even turned to my old valve data books and it's not listed in any of them, the only 'TV' type number valve listed being a TV4 tuning indicator. I have a feeling that there will be forum members who know this valve and probably have data and information on it and also for what purpose it was produced by Mazda in the first place.
Link to Radiomuseum page that gives filament voltage and current, but that's all:-
https://www.radiomuseum.org/tubes/tube_tv250.html
And some pictures of the valve itself:-