To specifically answer the question, one could say that the average musician statistically didn't know theory very well, if not at all.
Probably, considering the access to knowledge we have today, we can assume that the median knowledge was usually much worse, but it's just an assumption that is hard to evaluate: while it's true that today we can potentially study music theory almost for free and whenever we want, there's also to consider that we now have a huge amount of sources, many of which are absolutely not valid.
I cannot personally speak for that period of time, but I can recall the situation in the 90s and reference reports from colleagues, friends and family members that actually lived in the 60s. And I'd say that it was almost the same: yes, less was known, there were less sources, teachers or books, it was normally more expansive to get an instrument and have (good) lessons, but that also implies that there were way less people actually playing back then. By reading and watching forums, videos, groups and posts, what the average "musician" now knows is almost the same, they are just much more.
Actually, if we try to create a parallelism with todays popular musicians, they possibly know much less. If McCartney and Lennon didn't know how to play a B7 chord, there are lots of widely known (on social networks) musicians that don't even have a clue about what "B7" is, and that's because new electronic instruments make music production far more easy (maybe too much, I'd say).
This doesn't mean a lot in terms of music "capabilities", as two important aspects should be kept in mind:
- pentatonic scales are common in many music styles and ages, all around the world; you don't need to "know them";
- using a specific music theory "rule" doesn't mean that the user knows the theory behind it; consider spoken language: you can fluently (and correctly) speak a language even if you are not aware of the theory (grammar) behind it;
The following (rather extended, I admit) was to clarify the reasons for which the awareness of theory aspects isn't that important, especially for popular music; while it might seem a bit off-topic, I'll leave it anyway.
Pentatonic scales all around the world (and history)
First of all, while two "main" scales are normally considered the pentatonic scales (I'm obviously ignoring other pentatonic scales), it's common to actually consider the minor as the relative minor pentatonic.
Those intervals (no matter the root of the scale) are well known and used as they are simple and both easy to hear and play/sing. Many child tunes and nursery rhymes use those intervals, with the significant role of the minor third interval between the third and fifth (or the first and third for minor) which is very common even in the spoken language.
In the Kodály method, which uses those scales a lot in the ear and singing development, the music learning begins with the minor third (mi-so), then gradually adds the sixth (la), the first and, finally, the second. This pattern is easy to learn, both in recognition and reproduction, as is very close to the intervals used in the spoken language and introduces intervals based on their difficulty.
Obviously, there are many theories that try to explain the origins and spread of those intervals and scales (some may argue that they are easy and common because we're used to them), but, while those researches are indeed very interesting, I'd also say that it's more a matter of "the chicken and the egg".
Consider the blues scale, which is an expanded pentatonic scale, derived from "African scales" with the conjunction of western's music diatonic and chromatic principles. Or many of the eastern music (if you play around a pentatonic scale, many children would say that you're playing a chinese/japanese tune). Even music from the Middle Age through Rennaisance is reminiscent of pentatonic scales, with the proper importance of certain degrees in some modes (most importantly, the Dorian mode).
Using is not (always) knowing
Does anyone here know how well known such matters were back in the 60s amongst the average band member
Many popular musicians don't know a thing about theory (that doesn't make them less valid than others), and not only today musicians.
One of the main principles of popular music is that it is easy to listen and possibly sing (or play): it's easier to remember a tune that you can hum, sing or whistle, and this makes that tune more recognizable and easy to be shared and better know among people (hence, popular).
Popular music becomes such when it can easily "travel". This includes the music and the instrument.
I'm making a big oversimplification here, but consider that instruments that are easy to play and carry around (a guitar, a recorder, your voice), contribute more on the spreading of the music they can play.
For example, the learning curve for a guitar is, at the beginning, very steep (in the meaning of "it's easy to begin with"): it might take you less than a week to play some well known songs. And on a guitar, a pentonic scale is both actually very easy, and is also one of the first things you may begin to learn.
This doesn't mean that you know the theory behind the pentatonic scales (nor that you need to). All you need to know is how it works when you have to play it.
All this is valid today as it was in the 60s or even in the last centuries (and millennia, I think), and goes for almost any basic music principle.
Of course, knowing the theory behind what you're using dramatically increases the possibilities of what you can do, but that happens for almost anything: you don't need to know how the engine of your car works in order to drive it, but if you know it you might improve the efficiency of your fuel consumption, avoid damage to the engine, increase the life span of the vehicle, etc.