From the FIA archives in the Jaguar XJS homologation papers. They show the details when ol Tom pulled the scam of the decade homologating the pre-may type heads despite them being long out of production. "May" type heads were restrictive and designed for swirl to get better fuel consumption, great for the then fuel crisis times but not ideal for racing. When ol tom got this "erratum" through his jags jumped from 390 to 460HP. As the rules at the time said parts like heads had to be the currently fitted to the road going car how the heck did FIA rubber stamp it? Note the visible difference in combustion chamber design and the port size difference!

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