In the summer of 1970 I had a summer job programming the Atlas at the University of London. An amazing machine -- the tape drives would have looked right at home in an Avenger's episode.
Four years later I joined IBM, and my first job was to write the core of what eventually evolved into VM/'Passthru.
I should say that I never was directly involved in anything to do with virtualization on the Atlas, as I just programmed it in FORTRAN (FORTRAN V, actually, with recursive ALGOL-inspired block structure!)
I do recall learning later a couple of interesting facts about the Atlas: 1) I believe it was the first machine to use inverted page tables, decades before IBM trumpeted that "innovation" on its RISC machines and 2) as I recall, it took an interrupt to fire each hammer on its high speed line printer, thereby simplifying the control logic in the printer, but putting some significant timing constraints on responsiveness of the OS interrupt handlers.