I have an opportunity to buy a Dacor and a Sherwood AL80, both with current hydro and vis, for a cheap price. I checked with the seller and he says they were both manufactured in 1991, which I know is very close to the cutoff for AL tanks made with the shunned 6351 alloy. Most of the literature I can find says that only tanks made before 1991 have this problem, but I wanted to be sure. Are these ok to buy?