Your observation of this officer's side arm is spot on. I wonder if the photographer was drawing our attention to this anomoly since the gun is front and center in the photo. Hmmmm.
It appears I was not the only one impressed with Denver police weaponry at the time. From my own story "I clearly remember seeing a motorcycle cop drive by when I was in downtown Denver wearing cowboy boots and a six-gun." In picture 19A, of group 1, the officer is not wearing cowboy boots but I would be surprised to learn that his unsecured handgun was standard police issue.