Nice bike. I can tell you a bit about the derailleur. If you're used to modern bikes, you probably wouldn't notice anything, but back in the 70s, those SunTour Luxes were a hot item. They weren't expensive but they shifted better than most anything else at the time. They brought real quality, smooth, quiet shifting to the masses. I can see why your father did it. I bet he was happy with the change. Looks good on the bike now. Keeps the vintage look intact.