You're right. It is galvanized full length on the outside and still looks good after I stripped the rest of of the old non original paint off on tuesday, and then took the tank to my hydro facility (TÜV). They removed the rust from the inside and then hydro'ed the tank. I told them that WP was 150Bar and that the TP should be 250Bar which it passed with flying colors!
The only difficulty I had was when I insisted on them not stamping the tank with the recent hydro, they finally gave me a certificate and did not add any new markings to the tank!
I'm currently having problems finding a replacement for the original nylon INT sealing ring for the valve, it appears that I'll end up making my own using a knife-compass (used by coin collectors to cut the circles out of the heavy paper they use to display coins) and a sheet of NBR material.
Or does anybody know where I can get a few replacement soft nylon sealing rings OD .740" ID .385" thickness .100"?
BTW it appears that one of the prior owners was a cavediver

The original lead bullet burst screw also had a copper 3200PSI disk under it to efectively double disk the tank. I left it in since I would be very unhappy to be hit if the lead bullet ever let go in my direction, it could ruin my whole (hole?) day.
Michael