As far as I know, nobody can say for sure what optics came out of what company, across-the-board. Did some make in-house? Certainly, that is known. But there were optical shops putting out just lenses, prisms, and mirrors, and they were probably supplying everyone at one time or another, even the ones with in-house fabrication, if a large order came in and swamped them I'm sure they would just buy optics to fill the order.
I have some Towa 50x600mm that test very well. An old Milben is good, it is in every way a Tasco 6TE clone. And I have a very early 6TE OTA that tests very well. But I also have some that don't test as well and they far outnumber the really top-notch ones. More have come through my hands for testing that weren't of Swift or Royal Astro quality.
The thing to remember is that an imperfect lens in these small apertures can still produce imagery that all of us would consider excellent (see the first post in this thread). Very few people can say with certainty that an optic is good just based on the images it's produced, with nothing at hand for comparison. What finally turned around my thinking was the side-by-side test. A known excellent scope is used alongside an identical scope with a less-than-perfect lens. Both must use identical diagonals and eyepieces, and the images compared on nights of excellent seeing. It is only then that the difference is noticed, typically at higher magnifications and on subtle details. You have to swap diagonals and eyepieces to make sure what you are seeing is a result of the objective alone. (It almost always is.)
This backs up what Dave G. has been telling us. Experience and many hours spent observing aren't a guarantee that we can judge optical quality with anything other than generalities. The difference between "good" and "as good as it gets" can be very hard to judge with certainty if you don't have a top-notch reference at the time. The only way I know how to tell them apart sans bench testing, is the side-by-side observing test, and I've got nearly 52 years of experience at this now. It takes the bench test to remove all doubt, and see just how close we really are to having that "as good as it gets" objective if you can't do a side-by-side observational comparison.
But the good news is, those in the "second tier" are still eminently usable and some of them have given me terrific views. Just not the sharp views of low-contrast details. The rave-worthy stuff that a true 1/8-wave optic can produce. The "good" scope will show some nice details in Jupiter's bands, for example, while the "excellent" scope might show you those little blue festoons and other details not seen in the other scope.
The seeing itself is a huge factor in observational optical evaluation, especially without that reference optic. I like that bench testing removes the elephant from the equation.
Some of the old Towas that could make a better showing under DPAC are still leaps and bounds better than a lot of modern telescopes I've tested.