This is the original version of this tube. The earliest ones were all made in the USA by the usual manufacturers like RCA, Sylvania, GE and Tung Sol. Very early ones have rater large size plates. The "plain" version without the "A" suffix can only be used in parallel filament circuits, not series. This usually isn't a problem as series circuits are only normally found in TV sets. All of the tubes will have odd voltages other than 6 and 12. If all of the tubes in your set start with 6 or 12 (except for the 5 V rectifier) , then it is a parallel circuit.
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