Based on the two pics that you supplied, this steel tank was manufactured in 1966, and hydrostatically tested in June of 1992. The #'s and letters between the 6 and 92 identify the shop that performed the hydro. All portable steel (and Aluminum) CO2 tanks must be hydro'd every 5 years. My biggest concern with this tank is that the valve probably hasn't been removed in over 20 years. With the rust shown on the neck, there's a good chance that there's rust inside the tank as well - which could easily lead to a hydro failure.
Hydro's can cost anywhere from $20-$40, and you have to pay whether the tank passes or not, as the labor's all the same.
The chart someone posted earlier is for aluminum tanks, and they're not readily transferred over to steel. Most of the tall skinny tanks that I've seen around are 7 lb, FWIW.
As others mentioned, you're better off either buying a tank that's already been tested within the last year or two, or getting into an exchange program with where ever you're going to be getting your CO2 from, and let them worry about the tanks.
