> I'm guessing because they didn't have attached copyright notices as required in US law at the time they fell into public domain In IBM’s case, it wasn’t an o

> I'm guessing because they didn't have attached copyright notices as required in US law at the time they fell into public domain In IBM’s case, it wasn’t an oversight, it was an intentional decision. They intentionally put their core mainframe operating systems into the public domain, but they still copyrighted certain applications and add-ons. In later versions (like this one and its immediate predecessors), most new OS features went into copyrighted add-ons only, and the core OS by itself became more and more backward and limited. Then eventually, they came out with new versions of their operating systems where even the core was copyrighted. They did use other methods of “closing” software than copyright - much of MVS was written in a proprietary dialect of PL/I for which no compiler was publicly available. So even though they shipped the source code to customers, and the source code was public domain, nobody could compile it (unless you wrote your own compiler). VM/CMS (especially at this stage) was still mostly assembler though, so it was easier to modify than MVS was.