RK and Jeff,
Thanks to you both for motivating me to learn the year of manufacture of the Simson shotgun action. Per this link to European proof marks,
http://www.hallowellco.com/proof_date_codes.htm "Suhl, Germany: Month and year, 3 or 4 numerals as required, with forward slash between month and year from 1921 until 1930s, as: 5/24 for May 1924. Afterwards without slash, as: 1163 for November 1963."
So, the proof date was either 2/70 or 12/70, as the first character position has been over stamped with a "4".
When used to make the .405 rifle, the characters ".405" were stamped onto the barrel flats and as luck would have it, obscured the first numeral of the proof date. On one side all but the /70 was obscured, but on the other, 2/70 is clearly visible.
