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All Series 1 fuel gauges are "positive earth" gauges, but in fact are polarity agnostic (as are the later gauges).
However, when the wiring polarity changed from positive earth to negative earth in 1967 (Series 2a) the type of guages also changed from one that operated magnetically to one that works using a small heating element and a bimetal strip. It also needs a stabilised voltage (10V).
While both types work with either positive or negative earth, neither will work usefully with the wrong sender, and, in the case of the later one, the voltage stabiliser. The two gauges look like they are interchangeable.
My guess is you either have mismatched gauge and sender, or a later gauge and sender, without the stabiliser. However, if you are certain you have everything correct, the gauges will still misbehave if either the sender (and stabiliser) are not correctly earthed.
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