In my 65 year love with XK engines, I’ve never seen this one before. From a 1957 Mark I 3.4 liter engine head, there is some type of instrument sensor mounted on the boss between the #3 and #4 spark plugs.
It has a single terminal and was not connected to anything. This Mark I has a conventional oil/temp gage and no supplementary gages were installed.
The “boss” is a core plug to the water passages, so I would think somebody once rigged up an auxiliary electric water temperature gauge. Perhaps their regular Bourdon tube water gauge failed.
Ron had the right answer. On the original oil/temp gage the temp gage doesn’t work. The electric gage had been removed from below the dash. Here is a picture.
