Yeah, they certainly didn’t want to give away what the new characters looked like until the movie had been out a while. I received a textless version of that poster from the original SW fan club around 1981. I think that was the first time I ever noticed a poster variation as a kid.
I don’t think Yoda was added until much much later.
Didn’t KENNER hold back the Yoda action figure too? Not wanting to ruin the surprise.