The Pentacon Six metering prism is designed to take a 1.35 volt PX625, PX13 or equivalent.  These are now difficult to obtain (but occasionally available new on eBay).  There is also a 1.35 volt Wein cell that is occasionally offered on eBay. 

It is possible to use instead a 1.5V battery, but this will give sllightly incorrect readings – under-exposing a little.  The meter is designed with a bridging circuit, so that as the voltage drops from the original 1.35V, the meter continues to give correct readings.  However, it can’t cope with the higher voltage.  Tom Page in England has adapted my metering prisms so that they give correct readings with a 1.5V battery.  I imagine that any camera repairer with the repair manual for the prism could do this, as there is a simple adjustment within the prism, to give the correct reading.

Alternatively, you can run some tests and simply “recalibrate” the meter yourself by making a new index mark for the film sensitivity on the meter dial.