UX built to CC 3.1r4
Alias "defense-grade OS". EU implementation as ITSEC E6. Main precursor was DoD Orange Book. HP Labs probably had the first alpha of HPUX-10 to OB standard in the mid 90's but (officially) abandoned due to cost. Yes; it's "UX" - "nearly" Unix/Linux. There are versions today but not available commercially. Closest commercial relative - Linux!