Finding information on the cover artist Sax seemed to be a bit of a lost cause for some while. Jamie Sturgeon mentioned him to me some months ago when we were discussing an author, telling me that he appeared on the scene in the mid-1940s and disappeared in the early 1960s, having done numerous dust jacket illustrations, particularly for Hammond & Hammond.
Sax was also a cover artist for Pan Books in 1951-57 and illustrated Queer Shipmates by Commander A. B. Campbell (London, Phoenix House, 1962), although that is the only illustrated book I have traced for him. He also contributed two covers for Thriller Picture Library and illustrations to Everybody's.
The latter gives his initials as R. M. Sax, and a search of telephone records shows that R. M. Sax, industrial artist, was living at 12 Lindfield Gardens, Hampstead N.W.3, in 1951/68.
The address thankfully resolves the mystery of why I was unable to find any record for Sax's birth, marriage or death. A bit of good fortune meant that a wildcard search for people living in Hampstead in an earlier directory turned up R. M. Sachs, industrial artist, living at the same address. Sachs was also listed at the same address (sometimes given as 1/12, which makes me think it was a flat) in 1961/68.
Rudolf Michael Sachs' death was registered in St. Marylebone in 4Q 1969. He was born 22 May 1897. He had married Anneliese Bromberg in Hampstead in 1Q 1940 who died in Camden in 1998. Additional proof comes in the form of a shipping manifest for a trip the Sachs's took to America which lists Michael Sachs, 50, and Anneliese Sachs, 31, as travelling to New York in May 1948; they returned to the UK in June. Michael Sachs's occupation is given as "artist". Both he and Anneliese were born in Berlin, Germany.
This is still a bare-bones outline of a career that probably lasted at least three decades, but it's a start.