Here is the HAZMAT advisory (issued sometime ago) and the DOT regs. These are the rules that should be quoted, especially the latter.

HAZMAT Advisory... DOT Advisory AL 6351-T6 Cylinders.pdf
DOT regs... DOT Scuba Cylinder Requalification Regs.pdf

The source of this info was from the following Email received by Ian...

-------------- Original message from Allen Sanderson <allen@sci.utah.edu>: --------------


> Hello,
>
>Just stumbled across your newsletter in a web search. The article
> on the older scuba cylinders needs to be corrected.
>
> Aluminum cylinders that are made from the 6351 are safe to use -
> however they require an additional level of care that the newer
> cylinders do not require. This additional level of care are the eddy
> current tests. DOT issued new rules on this that are effective 01 Jan
> 07. That article should have noted this and NOT said they were unsafe
> to use.
>
> It did correctly note many shops are now refusing to fill cylinders
> made from 6351. That it their option but it shows that when it comes
> to the scuba industry and its ability to self regulate cylinders they
> fail.
>
> Sincerely,
>
> Allen Sanderson
> Salt Lake City