Considering they used these Exactra bubble strips on so many saber props, they obviously had easy enough access to them. Isn't there a possibility throughout all the publicity shots, stunt props, hero props, etc. - that along the way they may have lost a bubble strip here and there and had to replace it a few times? There are some publicity shots of Luke in ESB with a broken grip on his saber (by the silver screw).
My point is that they could have used both flattened sides and round sides. Just like us, when they opened the calculators, they didn't know which they would have.
Food for thought...