More Gull Puzzling

Here's an interesting gull that I found today with my dad Jim & Mike Freiberg at Cherry Creek Reservoir in the SE Denver Metro Area. Much about it reminds me of a potential 1st-cycle Slaty-backed Gull that Christian Nunes found in Boulder last November. Size was quite similar to a nearby adult Herring Gull. Getting a little zonked for much analysis now (here's my post to Cobirds for a little more description), but check it out & let me know what you think. As always, click images to enlarge.