I don't remember any models with PS-2 keyboards but had in my possession an old XT with DIN type or some proprietary connector. I believe it's possible that your keyboard was made by the same company who made similar ones for many companies including IBM. There's no telling where the parts came from. I think AT&T had some relationship with NCR at one time too. Anyway, if it says AT&T on the bottom, it was probably made for one of their systems.
I have this great old keyboard I found, but can't seem to identify it.
I'm pretty sure its IBM, its tactile and springy and resembles the old Model Ms.
Except the sticker on the back says AT&T with their very old logo.
Its PS/2, and I'm buying an adapter soon to use on my laptop, right now I'm typing this out on it and i'm flying its such a great keyboard.

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