It might be so old that it was designed with mercury-based batteries but environmental concerns have caused them to be pulled from the market. (Rightly so.)

Wein has non-mercury batteries designed as direct replacements for the mercury ones.

I'll bet if you check the Wein web site they might have someone who will tell you what to install.

Personally, I use a Minolta Autometer IV with my 30 year old RB and stovepipe finder.

Terry Thomas...
the photographer
Atlanta, Georgia USA

See some of my stills from the set of a movie here: