That "other unix" is probably UniPlus Unix, which we actually do have copies of, as well as its source code. It runs great on the FPGA-based Lisa too.
Here's the source code:
https://github.com/arcanebyte/uniplus
And here are the disk images, although I think you need to be logged in to download them:
https://lisalist2.com/index.php/topic,103.0.html