Yes, but it's meaning is often misunderstood. Our low pressure steel cylinders are DOT-3AA-2400 design i.e. their fill pressure is considered to be 2400 psi by the US Department of Transportation rules. When we talk about LP cylinders having a 2640 service pressure, that is always 2400 + 10% overfill = 2400 + 240 = 2640 psi. It does NOT mean a 10% overfill on the 2640 psi service pressure.

If 3AA design cylinders can pass a elastic expansion evaluation during hydro testing then they qualify for a "+" stamped next to the hydro date. That "+" means they may receive a 10% overfill. This overfill allowance was instituted to improve material capacity during war time almost a century ago, but the rule remains in effect today. New cylinders will always pass, but although rare do fail on later tests, especially those that have been consistently overfilled. Many hydro test facilities will not stamp the "+" on subsequent tests after the first, because frankly the DOT prefers the + not be used.