Duncan Granger asked: "Why vertical rear dropouts in 1973? Did they have some special advantage that made them better for a specific discipline (i.e. TT)? Why did they take so long to catch on?"
Duncan,
Another reason verticals were requested on time trial bikes is because they allow the chain stay to be as short as possible. The chain stay length does not have to include the slight amount forward a wheel needs to move to get the axle beyond the bottom of a horizontal slot to "drop out" of the frame.
Campy vertical dropouts lacked the nice look and feel of their forged dropouts in the raw. They were only 5 mm thick which meant their was a need braze on a round washer where the wheel would sit in the slot. Because a typical wheel axle sticks out more than 5 mm from the lock washer on the wheel, the result would be (if the dropout wasn't thickened) that it would poke beyond the other side of the DO so a nut or quick release would tighten up against the axle rather than the dropout itself. If the round washer was brazed on the inside, it gave more clearance when the chain was on the smallest cog to miss rubbing the end of the seat stay.
What Harvey said about the need for framebuilding accuracy when using verticals can not be overemphasized. The quality and accuracy of framebuilding fixturing during on-topic time periods does not compare with what is available today. Mostly it was straight edges and eyeballing to get it right. In addition, the pricing on British frames during this period required quick build times. Quoting what I remember Harry Quinn telling me when I asked how long it takes him to make a frame, "If I hurry, I can build 3 frames in a day". That was with a helper but even so that didn't allow time to fuss around. Even with the accuracy of today's equipment (like my Anvil or Bike Machinery jigs) care needs to be taken when assembling the rear triangle or a wheel won't center. One of my framebuilding students a couple of weeks ago had to redo a stay because he didn't take time to be sure everything was perfect before brazing.
Doug Fattic
Niles, Michigan