The thing on the weights form the 60's and 70's is that they are all with a stripped down unit. What I mean is book weights do not include AC, Awnings, Batteries in some cases, TV Antenna, or anything else that was optional at the time the trailer was built. I belive it was a way to try to survive the gas crunch and convince people that the Family car was alright to tow with.
This is the dirty little secret that most people don't figure out until the drag their "properly loaded" unit over a scale and see that they are overweight. Most of the Owners manuals from the period all have a grid in them for the owner to sit down and calculate the base weight of the trailer, before adding "stuff". I have studued them in every owners manual I have had, and have yet to sit down and actually figure it out.
Also as has been discussed on the forum the base weight of the trailer is assigned by the factory. Sometimes I think they give it a good guess, newer 19 FT bambi's that weigh 4200 lbs? I dont think so.
The engineers say it should weigh this amount with this interior, so that is how they plate it.
Just my $.02 worth, YMMV