I have 3 tanks hydroed in the early 80's, one in 83 has no RIN one in 86 does, none in the 70's have RIN. The change was probably about 85. Original hydros on PST tanks in the 60's and 70's were done by Cochran Labs, their symbol was an L in a C. Norris used an N in a diamond.
Notice the Norris logo at the top left.
Norris Cylinder - History | About Norris
You're talking PROOF MARKS, not re-qualification stamps.
Luxfer uses the arrow, Asahi the "circle Asahi", Metal Impact the "MI", Norris the "diamond N" etc. Thos are proof marks, not a RIN number.
The hydro shop doesn't have a proof mark, he has a RIN. The cylinder with the "?" wasn't taken back to the manufacturer for a hydro. The hydro guy doesn't manufacture the cylinder (qualify it), he "re-qualifies" it and that has nothing to do with the manufacturer.
Manufacturers use proof marks in the place of a RIN because they manufacture the cylinder according to the specifications they submitted to DOT for approval to sell the cylinder in the U.S.
A manufacturer doesn't "hydro test" the cylinder. Sure, everyone call it "the original hydro stamp" but it technically isn't. Manufacturers do a "proof test", which is only a stress test and doesn't measure elastic expansion of the cylinder. That is allowed under DOT regulations because the cylinder is brand new. There's no need to measure elastic expansion on a new cylinder that's never been pressurized before. The data submitted to DOT for a permit contains the cylinder expansion data, and that isn't going to change until the cylinder is used. So a formal "hydro test" to measure elastic expansion is redundant and a waste of time and resources.
4B, 4BA, 4BW, or 4E cylinders with a max service pressure of 300 psi and water capacity of 12 lbs or less may be proof tested instead of hydro tested. DOT "4" cylinders are usually LP and propane cylinders. Scuba cylinders don't fall into that category because DOT 4 cylinders are welded-seam cylinders. So the cylinder with the "?" couldn't have legally been proof tested except at manufacture.
When someone brings a propane tank in to be requalified, it isn't put in the water jacket and filled with water like a scuba cylinder is. The water jacket test does nothing but measure elastic expansion of a cylinder. With a DOT 4 tank, expansion doesn't matter. It's not measured or recorded. The propane tank is "proof tested". It's visually inspected, filled with compressed air to the designated test pressure, held for the designated amount of time, and if it isn't leaking, it passes and a new requalification stamp is stamped in the collar.
Proof test VS hydro test. There's a difference. A scuba cylinders "original hydro date" is a proof test date. That's all DOT requires.
