1938 M series petrol tank

[Rupert]

Would any lucky owner of a 38-39 M24 be able to take a measurement from the petrol tank please?
Am asking for a non-computer literate friend who needs the measurement between the filler cap and the edge of the instrument panel hole in the top of the petrol tank, so he can get the repair he is doing to that area spot-on.
I'm assuming the same tank was used on all M series bikes at that time, not including finishes?
Thanks in anticipation,
Rupert.