OM-1/OM-10 light meters questions.

Good day. I have two cameras: OM-1 and OM-10. OM-1 is modified with a BAT43 diode to use SR44 batteries. When used with the same lens and ISO400, I get different shutter speeds at the same aperture. Actually, you can see it in the photo: OM-10 offers 500/250 speeds, OM-1 at 250 speed gives insufficient exposure, at 125 it is already exceeded. What could this be connected with, and how can I make sure that the readings are correct? Should I use a third-party exposure meter application for testing?

Test it using a uniformly lit surface e.g. a wall. The bright light in your frame will give twitchy readings.

And don't forget that there are actually two meters in the OM-10. The one that controls exposure in auto mode measures light reflected off of the film plane, and depends on film being present for accuracy. The meter in the viewing system is probably not as advanced in many ways as the metering in the OM-1.