WAFFEN FRANKONIA-WURZBURG, founded in 1908, is still in business in Wurzburg, Germany, making sporting rifles, etc.
The proofmarks should help define the year/era your double was made, but from the gauge markings I would WAG that it was made prior to WWI.
Because the barrels are marked "16", and not "16/70" (2-3/4"), it has the older short 2-1/2" or 2-9/16" (65mm) chamber (all fired lengths) - and should NOT be fired with modern 2-3/4" ammo (even though unfired shells will chamber), even so-called "low brass" or target loads.
Proper, low-pressure 2-1/2" shotshells are available today from the likes of RST, PolyWad, Kent, GameBore & others.
The "16.1" (about IC) & 16.5" (about MOD) are the respective barrel's choke constriction measurements.
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