So I had another really close look at the flats using a jeweller's /watchmaker's magnifying optical visor. There is what appears to be an eagle with spindly dangling feet, then there is a mark 8BB and then what appears to be a coat of arms with a foot print in it and a diagonal line within the coat of arms running perpendicular to and through the footprint. Then there is the marking .500 NE 3" as well as DE followed by a 6 digit serial number.

On initial inspection, I saw the 8BB to be 88B as in the model of the rifle. My eyes just aren't what they used to be. By contrast the other flat is clearly and much more deeply stamped HEYM 88 B. GERMANY.

So, I suppose that 8BB translates to August 11 as in the year 2011.