[jasomill]

While implemented virtually rather than physically, two well-known (and very different) examples of systems that unify memory and secondary storage under a single addressing scheme are Multics (1969) and the IBM System/38 (1978).

Note that the present-day IBM i née AS/400 is a direct descendent of the System/38.

References:

https://en.wikipedia.org/wiki/Single-level_store

https://dl.acm.org/doi/pdf/10.1145/363095.363139

http://bitsavers.org/pdf/ibm/system38/G580-0237-1_IBM_System...

https://archive.org/details/insideas4000000solt/page/171/mod...