I am not sure why you say there is no information... a quick Google search returned a few interesting pages...
In this paper:
Progress in Metabolic Engineering of Saccharomyces cerevisiae - Nevoigt, Microbiol Mol Biol Rev. 2008
the author says:
The identification of the entire genomic sequence of a commonly used lager brewer's yeast strain, i.e., Weihenstephan Nr. 34 (34/70), represents a breakthrough in the molecular analysis of lager brewer's yeast.
So, it would look like 34/70 is just a catalogue number, with no specific meaning.
Curiously, according to the Wikipedia page on Saccharomyces pastorianus:
S. pastorianus never grows above 34 °C (93 °F)
So, I cannot exclude the hypothesis that 34 could come from there although, well, I personally propend for the catalogue number.
Other interesting links:
The paper about the S. pastorianus genome sequencing:
Genome sequence of the lager brewing yeast, an interspecies hybrid. - Nakao et al., DNA Res. 2009
An article comparing two different strains of S. pastorianus, 34/70 and 34/78 (again, catalog number hypothesis seems to be the most obvious explanation)
Molecular species of phosphatidylethanolamine from continuous cultures of Saccharomyces pastorianus syn. carlsbergensis strains. - Tosch, Yeast. 2006
The NCBI taxonomy page (entry #520522)