well, we know the XA91 was an M91ed equipped XA.
we know the TX and TT were both pre-XA two motor models. TX came first (this may be the 33 rpm only model), TT came after.
we know the TA was exactly the same as the XA and both were produced concurrently during the entire XA production run. when XAs had two motors, the TAs had two motors. when XAs had one motor, the TAs had one motor. when XAs changed to textured tops, TAs changed to textured tops. both could be had with solid walnut, or veneered bases. etc., etc., etc.
what we don't know is why bother with different designations at all between XA and TA. in the back of my mind, i still wonder if TAs were XAs that were returned to the factory under warranty as defective new units, then refurbished, and sold again under full warranty with the serial number kept, but the letters changed to TA. hence my speculation that TA stood for "try again". of course that is pure conjecture and there is absolutely no evidence to back it up.
something very interesting just happened that probably adds nothing to the mystery of 12ax7's table, but shows you never stop learning when it comes to ARs. today, there was a thread started about a 1964 XA find. i'm sure you saw it. original box, instructions, etc. all you'd hope for. plus good photo documentation. spec's out like this:
XA 37238 (1964)
motor: double
base: solid walnut
headshell: gray
plinth top: smooth gray
plinth bottom: gray
base bottom: slot vents, varnished
T-bar: notched
address: thorndike st.
warranty: 1 year
box: blue print
its been added to the database, and is the 3rd earliest serial number at the moment. what is interesting (besides it was a score!) is the serial number has a space between the letters and the numbers. just like 12ax7's serial number. what does that mean? i have no idea. but it is only the 3rd time i've seen that. it suggests there are other cases out there like it, and the idea the space is a typo holds less water.
also, the photos of the '64 table showed the location of screw mounting holes for the motors clearly. i pilfered a couple of his photos so 12ax7 can use them as reference to check his. it may be the textured coating just filled them in on his.
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