What does the 34/70 in Saccharomyces pastorianus Weihenstephan 34/70 stand for?
I went to the Yeastbank website at Weihenstephan for some info. The keyword here is "Stamm," which is German for stem, clade, clan, or strain. So, I would take this to mean that the 34/70 is an isolate (#70) of strain 34. Two of 34/70's strengths, according to the link above are it makes clean beer and gives a pleasant taste profile due to its low yeast-like aroma in the finished beer. I can attest to that firsthand, having had on numerous occasions the pleasure of partaking of Weihenstephaner beer in its home city of Freising.