I believe the differences you noticed are mostly due to the number of trailers produced in each of the years.
In the late 50's, they were pretty much made one at a time, semi-custom ordered by either a customer or a dealer. By '67 they were more production oriented, things were more mass-produced.
With that came more thought-out (sophisticated?) plans with bathrooms, but also compromises were made in the "hand-made" features like dove-tailed drawers. Your '67 probably had some plastic drawers or bins that may now be misplaced.
There were some minor differences with the "International" verisions in the 60's & 70's, but most didn't have anything to do with quality of construction, more like size of tanks, water filters, trim package, etc. and things that would be important for international travel like the Wally Byam or Airstream Co. Caravans.