I recently "attended" a meeting of the UK Computer Conservation Society on the subject of LEO (Lyons Electronic Office; Lyons was a bakery and cafeteria business that found itself designing and building computers), and mention was made of the multitasking capabilities of LEO III. There was some follow-up talk about the desirability of memory isolation for multitasking, inevitably followed by asking "who was first?". The contenders offered were Ferranti, English Electric, and LEO. I missed some of this discussion due to other calls on my attention.
The primary mechanism used in second-generation UK computers was datum and limit (base and bounds) registers. Programs run at virtual zero, the hardware relocating address references during execution by adding the datum, after checking that the address did not exceed the limit.
I'm going to post my own answer to this, which seems to be an approved SE thing to do, but I really am interested in other answers.
The question: what is the earliest example of the use of hardware datum and limit mechanisms for address-space relocation and isolation?
Secondary question: was the idea invented multiple times? It "seems an obvious thing to do", but that's easy for me to say, since I was educated on machines that had the benefit of the invention.