Try these:

Surplus Air with A Plus

Each 3AA steel cylinder receives an initial hydrostatic test to 5/3 service pressure. That test is documented on the cylinder crown by imprinting the month, year and tester's mark. New cylinder testers use a brand mark, while North American retester's must have a registered mark consisting of a letter and 3 numbers. When a new original test brand mark on a 3AA cylinder is followed by a plus mark (+), the cylinder is eligible for an additional pressure 10% in excess of the marked service pressure. A 2250 psig cylinder with plus (+) may be filled for the valid period of that test to 2475 psig.

Hydrostatic retester's are also authorized to retest a plus marked cylinder to determine whether the wall elastic expansion is low enough to retain the 110% fill. Most retester's choose not to get involved. Some facilities are unwilling to expend the extra effort or don't understand the process, so the steel cylinder owners seldom see a plus (+) after the original test. Since the plus mark is only valid for the duration of the hydro test, unless renewed along with the next retest, the cylinder must not be surplus filled. A hydro retest may, at any time, re-certify an eligible cylinder if he follows the prescribed protocol.


http://atlanticwreckdivers.net/Cylinders.htm


Or:

http://docketsinfo.dot.gov/reports/rspa/2003-06/030011.pdf