Those numbers are in millimeters and they represent the interior diameter of the unchoked portion of the barrel and the I.D. in the choked portion. The bore diameter of 16.9 mm is approximately 0.6653543 inches. The choked diameter of 16.8 mm equals approximately 0.6614173, which means that barrel (if unaltered) has a choke constriction of ~0.00394 inches or rounding off, 0.004 inches. Based on my limited knowledge, that would indicate a Skeet choke.
Here are some links off the old 16ga forum that you might find interesting:
http://www.16ga.com/forum/viewtopic.php?t=18620http://www.16ga.com/forum/viewtopic.php?t=1531The length for a 2.75 inch chamber will be 70mm, and you might see a stamping of 16-70 which would indicate a 16ga gun with a 70mm length chamber.
Have fun with the old Auto 5 - they were classy shotguns.