Understood, but why did they make it look like a horizontal dropout? A vertical dropout would achieve the same purpose, and make it easier to remove and replace the rear wheel. One of the advantages of a horizontal dropout is that they're much more tolerant of misalignment during brazing/welding. This dropout has neither advantage. Additionally, why is this derailleur so dependent on wheel placement when many bikes from that era had horizontals and didn't have this issue.

There's really no adjustability in this dropout. When I sold Hardrocks in the 80's and this dropout came out, we all scratched our heads. You just put the wheel in and slammed it all the way back, and they were all centered to our eyeballs.

I really think this was done because people's eyes were used to seeing horizontals, so they made them look like that. The fact that I could grind and file it down to look exactly like a regular horizontal dropout leads me to believe that this was the intention: