In Russia we have Rostechnadzor - government supervizing agency, who issue rules and recommendations for technical supervise.
And some years ago they issued new rule - all tanks older 20 years CAN NOT be accepted to hydro.
Some companies break it, and follow old rules - hydro passed - tank is good.
But really there could be a safety problem.
Two are dated the exact same date (9-1-42), and one is dated 1918. 101 years old and still in current service.
Can you show picture of it? It should be interesting!
I now question what would occur if a gas station refused to fill the gas tank of a 10? or 20? year old automobile ?
We (and may be you too) have similar limitation - it is forbidden to fill PLASTIC canister on the gas stations.
Here is the same, I think - safety reasons.
but the hydro in its current form is a rather crude method.
heh... There is different testing methods.
Some - just check - is is explode due hydro.
Some other - check geometry - they compare mid diamater before and after hydro.
Some - check how much water it press through outer of the tank (it tested inside another bigger tank, and check how much water goes out from space between tanks) - like DOT methodology use it and all above.
etc...