Yes, 12/70 is the CIP-compliant marking of a shotgun of "Cal. 12" as they say, or "12 gauge" as we say, that has a brass case or unfolded paper/plastic case length of 69.80 -2.50mm length, and chamber length of 69.90 + 2.00mm, i.e. "70mm" about 2-3/4" as we say.
The Hallowell note says if the year was 70, there would be no slash between the month and year numerals.
Builder stamped ".405" over the "12/70" which makes sense.
I am of no help with serial numbers for year of manufacture. That would take some research ... interesting ... I'll post here if I find anything.
