National Multiplex data cassette tape drive

Years ago, I owned a data cassette drive made by a company called National Multiplex, presumably long defunct. It was obviously a modified audio cassette recorder, but in which the internal electronics had been completely replaced. Unlike the prevailing standard for cassette-based data storage on micros in that day, this drive used saturation recording (NRZI, I believe) instead of a modulated audio carrier. It was a very functional replacement for a paper tape reader/punch, and ran up to 9600 bps using hi-fi grade audio tapes. I would be interested in locating one of these beasts to replicate the setup I had on my IMSAI before I sold the drive years ago. The drive came with schematics, and as far as I remember, the circuitry was rather simple -- a couple of op amps and a few gates. Alternately, if I can locate a copy of the drawings, I may attempt to reproduce the design using a contemporary audio cassette mechanism. Has anyone ever seen one of these? --Bill