Funny the question of undersquare frame came up - I am just working on Vintage Bicycle Quarterly 3, with a feature of what makes a good randonneur bike... and I recommend "undersquare" frames!
The thinking is as follows: In the 1980s, everybody dropped their bars, until they were many inches below the saddle. There is no advantage to this - you can get as aero a position by moving your stem forward, too. Furthermore, speed depends on being comfortable - if you ache, your power output drops!
The low bars worked OK for racers, who push hard on the pedals and thus don't need to support their upper bodies, as they are uplifted by the resulting force.
However, for normal riders, and those riding long distances, the power output is not enough to hold up their upper bodies. So stems are creeping up again, and we see sloping top tube, riser stems and the like. In fact, they are creeping up among pros, too, especially in long stage races!
In the old days, you'd ride a much taller (but no longer) frame. No need for trickery - your bars would be at the correct height.
So while I raced a 57 x 57 cm frame in the 1980s, I now ride a 61 cm x 57.5 cm frame in long-distance events. If I got a 1950s Herse in a 57 cm size, it would be way too small!
So when looking for vintage bikes to ride, think about how Cino, Ernesto, Alex, René or whomever designed the frame would have fitted you on a bike. Which makes it hard for some of us - few Italians were that tall back then.
Jan Heine, Seattle (whose 58 cm Rivendell combines a sloping top tube with a headtube extension to fit like a 61 cm Singer)