They all have the same circuit. The earlier ones with black painted cosmetics are more desirable/expensive cause they look cooler. Yes, some were made differently, mainly due to labor and part availability.
After 71-72, they changed from T & G pine with a screw-in baffle to completely glued marine MDF.
Great little amps, but I only have two. As far as worth, the going US rate is around $250-$350.
More for closet queens.