release dates of early microcomputer operating systems, incl. Intel ISIS

I've been searching for introduction dates of early microcomputer operating systems, by which I mean only operating systems that run on computers using single-chip microprocessors such as 8008, 8080, and 6800, but not the LSI-11, IMP-16, HP 9830, etc. Intel's ISIS operating system for their MDS was first released in 1975, but I haven't been able to pin down a month. I'm looking for a more specific date for that, and for the releases of any prior microcomputer operating system. On Twitter, @hotelzululima suggested Motorola MIKBUG, introduced in 1974, but IMO it's a monitor, not an operating system. Hzl also suggested Forth, which I also don't really consider to be an operating system in the traditional sense, but if there's evidence of Forth or a Forth-like language available for a microcomputer prior to 1976, that would be interesting as well.