I don't know the transition year for the Spridgets but for the Mini it was late 1964. That's when they changed from the magnetic gauges without the voltage stabilizer to the later bimetallic system with the stabilizer.
Things to look for are the gauge needle type and the sending unit resistance. To identify the early sending unit, disconnect the green/black wire from the fuel gauge's "T" terminal. Connect your multimeter between that wire terminal and ground with the meter set to measure Ohms. Use a coat hanger to lift the float. If the resistance increases towards 80 or 90 Ohms, you have an early sending unit. If the resistance drops towards 30 Ohms as you lift the float, the sending unit is the later type. Don't forget to hook the green/black wire back up.
For the gauge, look at the needle shape and behavior. A wide needle with parallel sides is typical of the later bimetallic gauges. The magnetic gauges use a thinner, tapered needle. When you turn the key to the run position, an early gauge's needle will "snap" quickly to position while the later bimetallic gauges will slowly climb to their reading.
In general, you cannot mix and match components because the early and late sending units have very different resistance ranges and change resistance in opposite directions as you go from full to empty.
Regardless, if you don't mind ordering from overseas, there is a solution for what you want to do. Visit the Spiyda Design website in the U.K. and consider buying their "Gauge Wizard". (I am posting a link below). The gauge wizard can match almost any sending unit to almost any gauge. That includes ones that work in different directions with different resistance ranges. The owner of the website says he developed this product to address mixing and matching gauge system components.
Link to
Spidyda Design Gauge Wizard
https://www.spiyda.com/fuel-gauge-wizard-mk3.html