Thush mufflers never freekin' had enough metal in them to last long enough to become "vintage." They were junk. Absolute junk. All you had to do, back then, and I did, was to take a Thrush in one hand and balance it on your hand, arm extended to get a feel of the WEIGHT.
Then take just about any other muffler in your opposite hand and ATTEMPT to do the same thing!!!