Back in the day a normal low gear was a 42x21. If you ran a 42x18 that was a "high low gear", so sort of like running a 39x21 nowadays. A quick and easy way to relate to a 42T is to subtract one tooth if you're using a 39T, so a 42x21 is like a 39x20. A 42x18 is like a 39x17.
I only used a 23T for hilly road races, like most people, and I would build the freewheel just for the race, then remove the unwanted 23T after. For climbs under a mile or so I usually had a 42x21 or 42x20, for climbs that required standing or that didn't, respectively.
Keep in mind that before ramped cogs most riders would customize their freewheels and, later, cassettes, so you could really build a super custom setup out back. No worries about lining up ramps etc - there were none. Remember also that riders only had downtube shifters. Some racers would use a right side bar end, but mainly they were crit racers thinking about the sprint. Usually if you were standing on a climb you committed to a particular gear and went for it. Sitting, shifting, and standing again would really kill your momentum. Bar ends didn't help much because you usually don't do long out-of-saddle climbs in the drops.
Eddy Merckx was interviewed at what appears to be the Campy 80th anniversary shindig. They asked him about compacts. "I wouldn't need a compact". Not only would he not use a 42, sometimes they were fitting a 46 or 48 INNER ring for the climbs.
Thierry Claveyrolat won the polka dot jersey in the Tour using a 45T or 46T inner ring, this in 1990. He helped Lemond in the Tour du Pont using similarly huge gears.
Right now I'm running a 53/44, left over from last year when I ran a 55/44. Normally I run 53/39 but my two sets of rings wore out so I borrowed the set off the tandem - when I got the 53x39 I kept the 44 for whatever reason. For about 10 years I ran a 54x42, 12-21, going to an 11-21 when 11T cogs became available.
53x42, 12-21 in this picture:
54x42, 12-21, a couple bikes later. Note the bar end shifter (just the right side), the Aerolite pedals (other than Looks the only clipless pedal really available for about a year or two).
