Adaptor to use old AT&T / keytronic keyboard with DB9 connector?
I'd strongly recommend sending Keytronic an email on this one. They're one of the few who will know for sure without disassembling and reverse engineering what you already have. They'd probably be thrilled to hear from such a fan of their work!

I went through something similar with a heavy IBM tactile keyboard from 1984. The cable unplugged on both ends and was lost in a move, so I was looking for the pinout of their slightly squashed RJ19 looking connector. It was manufactured by Lexmark, and they helped sort me out via email after routing me to their 'old man on the mountain' keyboard guy. Sadly, it died it's final death about a year later from a close encounter with a four year old. I still hit the thrift stores once in a while looking for another to pop up, but I've pretty much adjusted to my $9-on-sale keyboard now.