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It's 1 (one). It denotes the filament voltage which is actually about 1.4 V at 50 ma. There's a family of these 'one volt' miniature battery tubes like 1U4, 1U5, 1S5 etc. And there's 3V4, 3Q4 etc., which have a 3 V filament. And they're all 50 ma., intended for series-string service.
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