I've seen a wide variety of specifications and claims for dry-battery filament tube voltage. Most commercial-use tubes for battery radios (like the 3S4) are performance-specified at 1.4 volts, but most hearing-aid tubes, powered by the same kind of batteries, are specified at 1.25 volts. In fact, some mil-spec versions of regular radio tubes call for 1.25v +/- 0.25v - they are the same tube, only the spec has changed.
In the very earliest days, a rheostat was used to adjust dry-battery tubes to 1.1v, it was periodically adjusted to maintain the voltage as the battery ran down. (This is the origin of the 5-v tube too, with a rheostat and a 6-volt lead-acid battery.) In one RCA manual, the recommended range was 1.25-1.4 volts with a nominal 1.3 volts; an absolute maximum of 1.6 volts was called out.