It looks like that varied by year. I just looked at my wife's '84 720 next to her sister's '84 620. I just don't see a difference in the seatstay/seattube attachments. Looking at low-resolution images from the '84 and '85 catalogs, it looks like they fixed this problem in both bikes in 1985. Of course, I could just be missing the obvious. Maybe I'll have to get off my flu-weakened hiney and put a picture up so you can school me.

As far as the attachment being the weak link, it isn't all that bad. I'm a large guy (190-200 lbs.) and I ride hard and often carry large loads off-road. The longest any of these joints held up under me was 250,000 miles prior to failure. The shortest was 10,000 miles. One more failed after 100.000 miles and another hasn't yet failed after an unknown number of miles by other users and about 25.000 miles under me. Most riders would likely get many more miles out of these than I do. In fact, my wife has about 250.000 miles on hers. These old Treks are indeed great bikes and I would not hesitate to buy another if it becomes available. It just looks like your '85 620 is a little greater than my '82 720.