As others have said, the System/360 line wasn't even IBM's first generation of mainframes, but it was the first line of computers with an ISA as we understand the term: A document describing an environment for software to be written for independent of any specific machine, such that the machines would be implementations of that ISA and multiple different machines could all run the same software. IBM used this to make a whole line of different mainframes at different price points that could all run (mostly) the same software, to the limits of different machines having different amounts of RAM and some machines not having disk drives and some opcodes not being implemented on certain low-end models, just like how it works these days.
But that came about in the mid 1960s, and IBM had been making commercial mainframes for over a decade by then.
