I recently sourced Wein Cell batteries for my Leica M5. These give the correct 1.35 volt output and they replace the original, now no longer available, mercury oxide batteries. Here is a short review of my experience with the Wein Cell and also my thoughts on the handling of the M5.
http://macfilos.com/photo/2016/4/26/leica-m5-small-battery-company-wein-cell-battery-solutions
I would welcome views, particularly from other M5 users.
William