Use Fisher Space Pen Refills!
Great pens. I have the II, III, and IV. The first thing I do out of the box is toss the Schmidt refill and replace it with a Fisher Space Pen refill. Fisher supplies the Parker adapter that mates with the internal 'clicker' mechanism. Never had a problem. Oh, and customer service? I snagged a pocket clip once and bent it. Phone call ... couple of days later I was installing a replacement clip. No worries.