Peter,
I heard that Coopercraft have closed permanently , which is a shame . I suggest you contact a Jaguar Specialist and get the measurements of a early XJ6 caliper which have three pistons and a large pad. These calipers are for a solid, non ventilated disc rotor and commonly fitted to XJ6 S1, some Ferrari, Lotus, AC Cobra and 410 - 411 Bristol
The fixing bolt spacings of these are 3.5 inch which is common on many Girling calipers
When I fitted the Coopercraft 4 piston alloy calipers to my first 400 I had no caliper carriers . It was not too difficult to make a caliper carrier to set the caliper in the right position and right offset to suit the disc .
There is surely other companies that offer XJ6 S1 replacement alloy or cast iron 4 piston calipers.
Both of my 400 have 406 disc on the front, one with Coopercraft 4 piston alloy and the other with Girling #17 cast iron calipers, used on Humber Hawk & Snipe 1960 onward .These have two large pistons of 2.25inch diameter but a smaller pad than any 3 or 4 piston caliper.
Geoff
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