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The calibration issue is unfortunately not uncommon. I had two 100/2 Planars with that problem. Fortunately, it is easily fixed. If you strip off the rubber grip, you will see six screws. After loosening them a bit, with a proper screwdriver, you can recalibrate infinity focus simply by rotating the barrel and fastening the screws again.
Do the screws need to be precisely adjusted to avoid misalignment, or do they only hold the infinity stop in place?