WEs,
The earliest C-type heads (prototypes, and what was fitted to production C-type racing cars) and other special order applications pre 1954 did NOT have any cast in ‘C’.
As per my detailed overview, the earliest ‘production’ C-type heads as readily available on production XK140MC also did not have cast ‘C’, being something EXTRA added by the foundry from early 1955 (see my advised Engine Numbers range).
But 100% NO - C-type heads were NOT introduced nor available for XK120s.
After researching this red-herring to death, there are two only, and possibly a third (and maybe a couple more at most) XK120s that have been reliably confirmed as having second-type C7700 /C2 C-type heads fitted new by the factory - and these don’t of course have the large cast ‘C’, nor indeed were they originally painted red. So there is no way of identifying these from above, in an installed engine. All XK120 Special Equipment had an Engine Number with a suffix S added. This does not mean a C-type head, and that is I am sure the source of the confusion, as the suffix S on an XK140 Head does denote a C-type head being fitted (a third variant C7707/C3 head).
NO XK120 ever got a factory fitted ‘production spec’ C7707/C3 C-type head.
Roger