In Australia, in Concours circles bright zinc plating is an acceptable alternative to the original silver Cadmium (standard, not passivated). But our top restorations and indeed self-satisfaction owners do get things Cadmium Plated if original. Throughout Australia Cadmium Plating was essentially outlawed/priced out of existence because of environmental laws, more so than any toxicity concerns, however given Cadmium Plating is clearly known/acknowledged as being superior type of protective coating, there are special exemptions for Military, Aircraft and indeed Mining Industries where quality is paramount. Net result is there are a number of local platers that still offer Cadmium Plating, even if for a non-exempted automotive part…, and quite happy to receive and send parts through the mail, albeit I always prefer/like personal drop off/pick-up.
And I am aware that you can still get Cad plating done in USA, but no idea about NZ, Canada, UK or Europe.
In a Jaguar context, all Cad plating was the normal ‘standard silver’ Cad plating up until about 1972/3 when they introduced Passivated Cadmium Plating that has the yellow look rather than silver.
It is pretty consistent demarcation established when looking at V12 E-type (1971-1975) under bonnet originality, albeit that’s not to say there may have been instances of Passivated Cadmium being used on purchased in components from outside manufacturers/suppliers slightly pre-dating that, but I seriously doubt any passivated Cadmium used in 1950s and 1960s Jaguars.
But be aware, any spare parts made/purchased after 1970s, even if an XK spare part, may well have then been made with passivated cadmium, even if original part was silver cadmium…
XK Seat Runners … do what suits you