sachistu: Rubinsztein is an interesting spelling, and one that is new to me. In 1958 and earlier, his name was listed as Arnold N. Rubinstein in the South African Chess Player.
For some reason, in 1960, the South African Chess Player listed the name as Arnold N Rubenstein following his victory in the Western Province championship. There is a photo with the caption 'A.N. Rubenstein'.
From the tournament report describing his previous activities, it's clear this is the same person listed in 1958 and earlier. The reason for the name change (spelling) is unknown to me.
The Rubenstein spelling seems to have been picked up by some other sources i.e. Di Felice (Chess Results). However, that information is inaccurate.
In SACP, 1955 (p.150) the name is listed as Rubinstein (not Rubenstein, as given by Di Felice).
In SACP, 1966 (p139) again the name is listed as Rubinstein (not Rubenstein).
My speculation is the 1960 spelling of 'Rubenstein' is erroneous in as much as SACP reverts to the spelling 'Rubinstein' which it used in the 1950's.