The caliper interference occurs with the tapered backside of the pre-disc brake wheels. Almost all steel wheels prior to disc brake had a cylinder shaped front side and a conical/tapered rear shape to the rim, for ease of installing the tire. When discs came along, they simply made the back of the wheel cylinder shaped like the front, providing more clearance between the back of the wheel rim and the caliper.
Although the photo below is an early Radir mag, it uses an original pre-disc brake rim. I've marked the tapered area that normally causes the interference:
