Terry McCassey: I replied earlier to this query. They were fuel temperature gauges. The Conway engined aircraft had manual fuel heaters and every time a change of fuel tank feed was done the heaters had to be "toggled" to the required temperaure as the tanks could have quite different fuel temperatures on a long flight.
I think this function must have been automatic on the JT3D engines as I have no recollection of needing to do this with them.