I agree. Somewhere in the mid 60s, to maybe the late 70s seems to have been the time to trust their product sight unseen.
I have a big old Jack from 69 that has had three owners including my Dad and me. It still fits and functions perfectly, and snaps like a bear trap. It is a really heavy duty knife, and it has been used pretty well over the years, but no sign of joints opening, cracks in the scales, and it even has the original shield intact.
I still have the last CASE I bought from that era, in 1976. It is an odd copperhead, case length 4", and has a large clip blade like a jack, but also has an 1 1/2" pen blade. That was my main work knife for years, and while the large blade has lightened on the snap, it is still great. The scales are worn fairly smooth (used to be red bone, but years of sweat and grime have turned them dark chocolate), the pivot pin is now proud of the bolster enough to see it, and one of the pins holding on the scales has lost its head. Overall, the knife is still solid as a rock and has no gaps in between springs, liners or the blades. Oddly, no blade wobble at all, even though there has been enough use of the main blade that it is rolled back a bit on the spine where it contacts the spring when open. And I love the hollow grind on the main blade as that makes it a slicing son of a gun.
Robert