On my work bench I have 2 of these, one is '34 69' and the other is '13 70'. Unless there were different date formats, I think the "month, year" format could be ruled out. If it was "Julian date, year" I would expect to see many with a 3-digit date, followed by year, but have never seen an xxx yy stamp, only xx yy (anyone else seen an xxx yy stamp?). So maybe it's "week, year", and if so, is it production week of model year (like gas tank date stamps) or calendar year weeks??