[QUOTE=bandersen;3067987]
I dug up some datasheets on the tubes and they both show 1.25 +- 0.2 for the filament
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If I remember correctly, it had more to do with the design of the filament itself (support, protective coating, etc). For instance, a HV rectifier subjected to filament over-voltage (and resultant high temperature), may have the nickel support rods burned away leaving the filament intact but with insufficient support. Tube engineers often changed the internal makeup of tubes to help simply improve their useful lifetime, which is what I believe to be the case in the 1B3/1J3.
Of course, I'm trying to remember the contents of an article I read years ago.
I can't find the article discussing the 1B3/1J3 differences, but I did locate the one about why HV rectifier tubes (of the 1B3 era) failed. It was also the one that tells of the advantages of replacing any of the 1B3 family of tubes with a 1AX2. It came from a book of tech tips published by CBS-Hytron.