> when was variable witdh spacing introduced (rather than cutting types
> in varying widhts as Gutenberg did)?
But since Gutenberg (we presume: his type doesn't survive) types
always had varying widths: until the manual typewriter and
"monowidth" type that emulated typewriter characters.
There is another question that overlaps the one you seem to be asking.
To let lines of text have equal width (to let the type be locked up
securely), you need
(1) to put spaces of varying width between the words; or else
(2) you need to put spaces of equal width between the words and then
put spaces of varied widths at the end of each line (the system known
in English as unjustified setting, though in the larger scheme of
things, it is also "justified").
One recent writer, Peter Burnhill, has discussed this in much detail
in his book *Type spaces: in-house norms in the typography of Aldus
Manutius*. He concludes that Aldus, around 1500, and presumably other
early printers, did use spaces whose widths belong to a system of
ratios. And, in effect, this was a system of typographic measurement
-- two hundred years before such a system is usually considered to
have come into use.
A modest advertisement: Hyphen Press published Burnhill's book last month.