I’m thinking it’s a catadioptric (mirror lens) mostly because of the description of FL and fixed aperture; and because th small highlights in photo are a giveaway. Your closeup of Sandhills cranes IS tack sharp and it’s the only one in which you can clearly see an eyeball on which to focus. With a 500 mm, handheld, I would guess that at 25 -30 yards your only “sharply - defined” view of a bird’s eye would be when it’s exposed directly to the light source and the bird is static. If you are panning a BIF, and the bird’s line of flight is anything but parallel to the direction of your panning, his eyeball is constantly moving through your relatively shallow depth of field. Even if you can see the eyeball, the bird will be moving in and out of the depth of field. Hard to get into focus and take the image at the same time… just remember when panning do not stop panning when you trip the shutter… follow through (it’s like shooting a shotgun; if you stop panning you’re already too late.)
The good news is that even using a mirror lens, you don’t have to pay to process the film and the delete button can be your best friend. Higher ISO’s will be your friend, but everything is a compromise. Your cranes are the proof of the quality to be had, but they are taking advantage of closer range, slow action, and direct lighting and fall within the capability of your camera body to adapt. Keep shooting. Good lessons to be had on this outing don’t quit looks like you’ve had some fun… to be continued….?