The 16 in a diamond is a pre-1924 mark simply meaning 16g. There are other marks on the action and barrel flats that provide more information. Smokeless proof is indicated by a stylized Lion over PV stamp.
This might help
http://www.shotgunworld.com/bbs/viewtopic.php?f=5&t=295608
The barrels are no doubt Twist, and no one on the internet can tell you if they are safe to use, with any load.