School me on Rolex magnifying windows

[ssmdive]

Met a guy to buy a Rolex Explorer II. The watch looked good to me, but when I looked at the mag window (is this what people call the cyclops?), it seemed to have a defect in the window. When you moved the watch from the 3' to the 9' position there was what seemed a defect in the bubble. This defect would present by shrinking or enlarging the date as you moved the watch from left to right.

The seller claimed that could be normal and the result of damage, but the bubble felt smooth. He said that you could swap just the bubble? This seemed strange to me. How would you even do that? Would that be the correct fix?

In the end, I decided to not buy the watch. But I wanted to learn more about the bubble (Cyclops?) and if it is actually possible that the watch was real and that is a possible defect, if the watch might have been real, but with a non-original bubble, or if it was possible the watch was fake.

I have no pictures.