A few years ago, I bought online what I thought was a replacement heater valve for my X-300. When it arrived, I realized it was the wrong one for an X-300, but since i had an XJS at the time (Supercat, my '95 4.0) I kept it rather than send it back on the chance Supercat might need one at some future time as well. This heater valve is the brass, Rube Goldberg-looking contraption that was the model for XJSes for many MYs. Funny thing is that Jag apparently “upgraded” the original issue, which came w/o a mounting bracket, to a newer version which comes with a mounting bracket, circa '93 or so. It looks like this bracket is designed to possibly mount the valve to the firewall rather than just have it stuffed down in the area behind the firewall end of the valve/camshaft cover (like it presently is on my '94). However, both my '94 and, IIRC, my '95 did not have mounting holes on the firewall for such a bracket. So, why did Jaguar then decide to put a bracket on it? 
In any event, I wouldn’t be able to mount mine to a firewall as for some reason it was able to go through quality control despite one of the pair of mounting holes not being drilled/stamped into the bracket. 
IIRC, the heater valve on the earlier cars – like my '83 – was mounted on a bracket.
I have 76 and an 89, both valves on brackets mounted to firewall.