We are talking about the world of fixed gain 40 dB preamps. Before the advent of high output mics and loud bands, these seldom overloaded. Let's not forget that RCA introduced the 'preamp' as we somewhat know it today to get the 'new' ribbon mics up to the level that condensers and carbon mics naturally presented to the bus. Then the ribbon took over for a long long time, and the preamp became a mandatory part of the equation.
One made gain adjustments after the preamp, not within it's gain structure. Broadly speaking, preamps were tools to get normal mic levels up to mixing bus level, which was typically -20 dBm. This is before the time that the mic amp and the bus amp might well be the same thing, and redundant to one another.
So, the preamp overloaded, or it didn't. Cut and dried, almost. If it did overload, it probably only did it on occasional peaks, the sort the band never presents during soundcheck, sitting on the launch pad. If it was obvious it would overload, the only real option was a front end pad that irrevocably subtracted from potential output level. Not like today, where you pad an overloading input stage and turn up the NFB based gain loop to compensate.
Add to that mix the variables in pad theory. On portable broadcast and other cheaper mixers you find 150 ohm T pads feeding a passive mixing bus, which feeds a single amp stage, so you have the losses occurring at mic level, rather than high level. Add to that the impedance issues of paralleling a bunch of mics and T pads into a single 150 ohm mic input stage. Or consider a console with discrete input amps, one mic to one preamp. What kind of pad might they have used? Easily a matching pad rather than a bridging pad, which may well change the sound of the mic radically. If one is near the edge of overload, but thinks it possible to get away without a pad, rather than change the sound of the mic negatively, one will probably take the risk. Especially with poor monitoring that is far better than what the man at home listens on. Oh yeah, you probably don't have a pile of inline pads laying around in the first place.
Lastly, remember they didn't live in a post-modern culture that constantly re-examined the cultural past. These recordings are now fully out of the context in which they were birthed, disposable goods to make a quick buck. Move on to tomorrow with the next disposable good.