Canon fine-tuned the meters in these cameras by adding a small bit of neutral density film between the lens face plate and the meter eye. The glue they used so hold the film in place is water soluble, so this film sometimes comes off or gets lost, and the result is under exposed images. You can compensate a bit by adjusting the film speed lever, but with the way the light baffle is designed, you would be better off adjusting one full stop and setting the speed to 100.
I have compared 1.5 volt batteries and Wein cells in a couple of Canonets, and noticed no difference in the meter readings. The meters in these old cameras are not especially precise, and should only be used to get a ball-park figure.