The detail in question is the fact that he has no feathers below his sternum. When I first saw the bare spot I thought, oh dear, poor guy, he's loosing his feathers.
Wait! Might he have a brood patch?
Yes indeed! Not only do Red-belly pairs share egg sitting responsibilities but they both loose their feathers in the "nest sitting egg contact area" and develop brood patches.
And if he didn't have a brood patch you would possibly be able to see why a Red-bellied Woodpecker has its name.
Now, when I was training and looking at Red-bellied Woodpeckers in the bird skin drawers, none of them had the long thin reddish area below "the waist" you can see in the photo below. I had even read that there was a only slight diffused "reddish hue" on their underside which could only be seen sporadically in the proper light.
Not true. There are dozens of photographs of the "red belly" these days.
Above is an example of the red belly on a female. All I can figure is that that patch must fade to near nothing on a preserved bird.
There is just no substitute for live birds and modern photography to get the actual scoop on any number of things concerning wildlife. As well as leaving them in the wild for as full a life as they can muster.
Happy birding!
Donegal Browne
P. S. Don't worry I never killed anything. (Otherwise I would have had to do theatre full time.) By the time I was in training we used road kill to learn how to preserve and stuff bird skins.