Both tubes are electrically identical. The "A" is a slightly more rugged construction which results in lower output microphonics (and the "WA" is a military spec version which is made for high vibration environments). I cannot see why they would not be interchangeable.
The letter indicates a "later and modified version which can be substituted for any previous version but not vice-versa" according to my GE tube manual. It also says the letter does not convey any information about the nature of the modification.
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