Originally Posted by LarDasse74

There aren't too many bikes 'designed around' 20mm tires. Modern road bikes tend to have extremely tight clearance - certainly tighter than any Nishiki Olympic from the 80s - and you'd have to search pretty hard to find a 25mm tire that would not fit on most. 28mm tires are a different story.

That is almost definitely a frame designed around 650c or b wheels. Perhaps the previous owner swapped to 700c and used 20mm tires.