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^Yes, the ring holds the rear element in place. As you approach infinity, the rear element gets closer and closer to the mirror. When shaving, you have to be mindful of the arc path of the mirror and where the retaining ring is seated when fully tightened. I think other people may have gotten away with having it clear at infinity with just modifying the retaining ring, its a bit hit or miss.
The retaining ring is also brass, which isn't as strong if you file away too much of it. Sharpies markers and a lot of patience is my approach.