Mercury 1.35 volt batteries are no longer available in the US, but many are selling an alkaline so-called replacement, which is 1.5 volts and will change the meter reading up to two stops. The problem with alkaline is that the voltage changes as the battery gets older.
I think the best thing is to buy an MR-9 adapter sold by C.R.I.S. This will last indefinitely and can be exchanged from camera to camera. Link to C.R.I.S.