update since the question was clarified later, some parts of it no longer fit.
However that probably isn't Endor because the Ewok film showed Endor as light gray as seen below. Not red.
According to illustrations in "The making of Star Wars: Return of the Jedi", it was indeed the forest moon of Endor. Or simply "Endor", as the name of the moon kept changing during production.
Here's the books illustration titled "Chris Evans at work on a matte painting of the forest moon of Endor":
And here is the shot with the planet looking nearly identical to one in your question, from a series titled "Final frames of the first five matte paintings of the film, by Pangrazio (#1=the curve of the forest moon)":
- Additional evidence that it is not Endor the planet: the planet is a gas giant. This doesn't look like a gas giant.
I'm still looking but I don't think Endor (the gas giant planet itself) appears in the film
Please note that the iconic image gracing both Wookiepedia page as well as this SE question is as far as I know sourced from Ewoks film (Caravan of Courage?) and not ROTJ!
