The wide-mount stacking side shells were produced in the US at least as early as 1956-57, and possibly earlier.
I found one on a base that had "swivel domes" feet (only produced for one year in 1956) and was definitely original to the shell. The white plastic glides feet began production in 1957.
No markings or embossments on the shell usually means at least pre 1960.
But I am not very well versed on the European production dates.
Hope this helps!
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