I would call it pure RES but I am also no expert. I see nothing that leads me to believe it is a hybrid.
Here is a picture of a RES I took out of my forage pond last night by cast net. Notice it has a very thin red opercular tab. Most of my RES have much more prominent red/orange borders. But I would also call this a pure RES. I see no other characteristics to make me think it is other than RES. There is just a lot of natural variation within species.
Be interesting to see what others think of both of our fish.