My girlfriend worked for Amdahl in the 80's as the QA automation lead to test the Unix kernel for their systems:
And, since she was one of a few hundred people that had actually done this kind of work, she later worked for Tandem doing the same thing on the Tandem NonStop UX OS:
While it is true mainframes didn't start out with Unix, they have all since been re-designed to use it - and for more than 20 years, they have been nothing else.
Today, mainframes are no longer sold, but maintained via service contract. If you want the modern equivalent, say an IBM Z server, it's all highly-customized Linux goodness.
But, if you want the real thing, just lay down the PO number for Linux on IBM System z:
The HP NonStop server too (formerly Tandem); (now) a highly-customized POSIX-compliant OS:
Although IBM and HP would have you believe they created an entire OS, out of the mist, as they did with all other computing. If creating OSs was so easy, there wouldn't be just 3 (Win/Lin/Mac) contenders; really there are only 2 as Linux and Mac are more brothers than anything else.
Your history is splotchy at best; messy.
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