Just got back from FRance..and the worst Tour de France that I've watched since I started way back in 1952. Even the Luxemburg - Schleck fans - with whom I was staying thought that there was something not quite right about the Ventoux stage! Fignon, commentating on French TV stated that there were "conneries" going on...meaning some sort of hanky-panky...race fixing.
Anyway... certainly UK framebuilders in the 50s and onwards tended to use the standard horizontal drop-outs because they tended to make the bikes much more versatile. It's as simple as that. By pulling the wheel further back in the drop-out slot the rider could use fatter tyres as the gap between the chain-stays widened as they got longer.
You should remember that in those days and for many afterwards many club cyclists would only have one "best" bike. If they were lucky they might also have a road-track model, also with horizontal drop-outs, but rearwards facing of course, for track racing and for riding on fixed gear in the winter.
It was quite normal for riders in my club to laod up their best bike on a Friday, complete with mudguards and saddlebag, and then ride out to stop the night at the nearest Youth Hostel to the race that we intended riding that weekend- usually on Sunday morning.
At the front end of the bike we would carry our pair of racing wheels, fastened into a pair of Cycle wheel carriers, with the upper part of the wheel fastened to the bars by toestraps to cut down vibration.
At the race we would dismantle the guards, the saddle bag etc, remove the wheels which would normally be shod with 27 x 1.25 tyres, and then replace them with a pair of 700c sprint wheels with much narrower tyres/tubulars.
The chainstay length on our bikes would often be about 43 inches to the back of the slot..so we could get the fatter tyred genereal riding wheels into the frame..and then with the racing wheels with their thinner tyres, we would push those as far forward in the slot as possible to obtain a shorter wheelbase. Some of those drop-outs had long slots!
The other advantage of the horizontal drop-out is that it enabled wheels that had been slightly buckled ie a broken spoke or similar to be ridden home..by puling the wheel further back in the slot.
Anyhow..that's how I remember it. Oh..that long prong on the Super Champion drop-out was there to have the first two fingers of each hand hooked and wrapped around them while the thumbs located on the wheel nuts. Using a slightly pivoting pushing action in a clockwise direction with both hands it was much easier to remove the wheel from the frame.
Norris Lockley..Settle Uk