Another gun with a similar mark, found with a Google search for 'WJN shotgun barrel mark'.

http://i.imgur.com/MGd2W4j.jpg

.776 makes no sense as a bore for a 16 gauge.

Perhaps it's barrel length in Meters?

Page from whence this came, with additional info:

https://www.reddit.com/r/guns/comments/30zyba/firebombing_nazis_and_shadily_procured_war/




"The price of good shotgunnery is constant practice" - Fred Kimble