Carla's Breasts

You're very close to the facts. It was all explained in several film magazines years ago.
The prop department created polymer body mold cast of Barbara Hershey but no head. The head was supplied by the actress herself.
The bed was actually mounted on top of a false floor platform. Barbara Hershey sat propped up under the bed and the prop torso/body was drawn up to her neck. Special effects were relatively primitive by today's standards. Barbara's long hair was drawn up and around her neck to conceal the juncture of her neck to the prop torso and the prop body.
Underneath the platform hid several prop department technicians. They held strings that connected above to the underside of the prop torso's faux breasts. When the technicians pulled the strings at random the effect was to depict invisible fingers grabbing Hershey's breasts.
It looked real enough. But you only need to watch the invisible hands/fingers breast fondling movie sequences several times then you'll start to realize it's not quite accurate after all. It would appear a non-human hand consisting of just four fingers mounted on the corners of a square palm is pressing down on the breasts. The special effects ignored the true ergonomics of a man's hand, namely that it is asymmetrical and has an opposing thumb. Grab a water balloon and pretend it is a woman's breast and you'll quickly understand what I mean. The movie's breast-fondling sequences aren't accurate after all, but this was not apparent at the time of the movie's introduction. And as I mentioned, special effects were advanced in 1981 compared to 1951, but now primitive compared to 2015.
There's going to be a remake of THE ENTITY. When it comes out, no one knows. But it would be interesting to see how far the movie's producers and director take the invisible assailant scenes. It's a fine line to tread because the advanced movie prop technologies available today could produce a more realistic female prop body and miniature robotics, sensors, and motors could produce much more realistic special effects.

Jeff, if you can find it, watch Box Car Bertha with Barbara Hershey - she is naturally quite hairy (though not as much as her prop department body double). Also, I was in college in the 1980s and based on my dating experience, natural pubic hair was still considerably more common than shaved. By the way, the majority of Playboy's Playmates from the 80's still had their natural lower body hair.

Colonel Angus: Well thank you Miss Anabelle. And if I overstay my welcome, just tap me on the head.

Lol what are you talking about? It was the 80's! Every girl had a giant bush. Talk about annoying lol

Yep it was some bush.

I dont even remember seeing her pubes. Why would they...it would crush the MPAA rating.

Back then I got lucky and my GF let me shave her. It was great!
Pubes are making a comeback.