A friend of mine has just bought a used Model 586 and it is not the same as the fairly new one I own. His version looks older, and has what looks like a gas port in the top of the barrel, between the front sight blade and the muzzle of the 6 inch barrel. It also has a small grub screw in the trigger blade which can be used to adjust the let-off point. The port looks original, but the trigger adjuster could be an add-on. Can anyone shed any light on these differences and possibly date my friends gun.

Many thanks,

Ian