Actually, if you can get this frame, I could figure it out. It's when 3PO is saying "or smashed into who knows what", R2 is turning his dome but hasn't started moving to the right yet.
I did a Print Screen of this shot from one of your clips, and I've figured out how much it's cropped on the top, bottom and left by brightening it, but it's still so low-resolution and lossy that I can't make out any detail on the right side of the frame.
Here's what I've been able to figure out so far. The blue box is roughly equivalent to the DVD/Blu-ray framing, the green box is hypothetical 5% cropping, and the red box is the Senator framing. The right side of the red box is an estimate based on Harmy's comparison:
I know from Harmy's comparison that the Senator screening was framed considerably narrower than 2.35:1. It seems to have been about 2.20:1 (which makes sense; I believe that the Senator's screen was intended for 70mm). The cropping on the top and bottom is about 9%, but the cropping on the sides could be as much as 16%; that's only taking into account the picture information in your clip, it could end up being more once I compare it to the full picture frame.
I don't think that the film was cropped that much in the vast majority of theaters. As Mike Verta has explained, the screening was cropped close to hide platter damage on the edges.
If anything, the Senator cropping is probably equivalent to the absolute maximum amount you'd have seen in a 70mm showing. 70mm prints were already cropped to about 2.20:1, and with a additional 10% cutoff, you'd probably see about as much of the frame as at the Senator.
I don't believe most 35mm screenings would have taken off that much, at least if they had a 2.35-ratio screen. I'd wager that the Senator screening is probably the closest cropping you'd ever see in any theater in '77, either 35mm or 70mm, unless maybe they were one of those venues that had a 2:1 screen and showed everything at that ratio (and this did happen).
If you can provide a scan of that frame from your print, I can nail down the Senator cropping.