In
1966, the sequential build numbers for the double bed arrangement begins with 500, meaning that between the kitchen and bath, you have a double bed on one side and cabinets on the other, as opposed to having twin beds on both sides. If your trailer had 2 twin beds, it would be 076. You have a double bed, therefore 576.
Your trailer is the 76th double bed Overlander of 1966 from Ohio.
There is no added value for an early 1966 production number.
Generally speaking, any year, mostly there, rolling Airstream, without a rusted frame, and having decent skins, has a rebuildable-core value of around 5K. Leaks, bad windows, dysfunctional appliances, freeze damaged plumbing, antiquated electrical, and floor rot, are a given. For any vintage year, as the trailers get shorter, they become exponentially more valuable.
The crack in your tank nipple might be able to be polyethylene welded. If you can solder, braze, Gas or Tig, you'll figure it out with the aid of the You Tubes, and you have a chance of an easy,
Cheap repair.