I really think that 40-45 lb bows were most commonly sold back in the day because most brand new bows were bought by brand new shooters.
My first Bear dealer convinced me to buy a 40+ lb Super Grizzly in 1972. He told me it was actually 45 pounds. I was a sprightly lad (about 6'1" and 190 pounds)and really wanted a 50 pounder. I reluctantly went with his advice, and learned to shoot with that introductory weight. Guess what? After a few months, it was easy to shoot. Too easy.
What was I supposed to do? ?????
Eureka! I should go back to the store and buy a 50+ pounder from the same helpful salesman.
What happened instead? I lost interest, and retired the Grizz by the time I was 30.
Years later, on a whim, I dust off the old Bear, buy a new string, order a half dozen arrows, and begin to shoot again. My Bear dealer is long gone. I don't realize it, but I am a trad archer.
Now the internet has become the virtual campfire for vintage archers. I read good, bad, and questionable things about trad archery. I buy a 55 pound Super Kodiak. It is comfortable to shoot. I find other bows that range from 35 to 55 pounds. The lightweight bows feel like toys, but of course if shot "just right" would kill a T-Rex.
One day a 60 pound Howatt Hunter arrives at my door. Zip. Zip. The arrows fly like laser beams. The bow, though, felt a lot more than 5 pounds heavier than my Bears. It was really hard to anchor at full draw. I really never felt comfortable with the bow, and now it lives in France.
A few years go by. I picked up a 60 pound Grizzly, and it shoots fine. I work up to a 70 pound Super Diablo, a 70 pound Kodiak, a 65 longbow, and several 58 to 64 pound recurves and longbows. For some weird reason, the 70 pound Super Diablo does not stack like its 60 pound cousin. Was it the bow?
In the 21st Century I am no longer exactly 6'1" (gravity takes its toll after 57 years). I am no longer a sprightly 190. Somehow 45 extra pounds are clinging to my lean, rock hard, muscular frame. Could it be something I ate?
Today a 55 60 or even 65 pound bow is comfortable, and the 70 pounders are great shooters to keep me honest. I never shoot 300 to 1000 arrows a day. A few dozen shots - one at a time, seems to keep the rust off the skills.
The so called heavy weights release like greased lightning and shoot flat as a skillet. I am trying to keep my skills honed for that "one shot" and any bow under 55 seems a little iffy.
So now, consider this. A bunch of those mint condition 40 pound bows share the story of my first Grizz. People bought a learner bow, stuck it in the closet, and either moved up or out of archery as the compound craze hit. These old mint bows are probably flinging more arrows in 2009 than they ever did.