Had one of them brain things that sparked a recall that brung back some thoughts about the mark you found on your cylinder. If the shield is the same shape as the Federal Seal there is a good possibility that it is the DOT stamp of approval. I was unable to find a direct reference to this idea but decided to look a little further and boy did I find a mess.
From what I found your vessel is not a cylinder, it is a tube, specifically a ground tube as it isn't used to transport gases over the highway. These are built under the same stamp as the transport cylinders except installed to be used for on site storage.
It seems that when the use of seamless steel tubes to transport high pressure gases over the highway was proposed the ASME didn't want any part of it so the DOT took it on to have some control of the design. Apparently the DOT just considered them just like a standard cylinder only longer. There were also rules on the transport trailer design.
The current regulations are sort of, kinda, sort of, convoluted as there is the DOT regulations concurrent with UN regulations that is controlled by the CFR xx-xxx. The ASME has gotten involved to the extent that you can now purchase a tube with the ASME stamp or the required DOT. One thing I couldn't ascertain for sure is will an ASME design meet the DOT criteria. I think all states exemtt the DOT stamped tubes from the local Boiler and Pressure Vessel Regulations.
I didn't completely check The CGA site. It Appears that your tube was made the DOT Class 3A or 3AA standards. The CGA site has information on the the testing and requalification and the life of such vessels. I believe the transport and ground tube have and indefinite life with testing.
The cylinders I mentioned in previous post we had made would come under what the trade now calls Hydril cylinders. Which again can be ASME or DOT stamped.
Here is a good paper on storage of H2 that covers essentially the whole nine yards, but no first down.
Here is another manufacturer of said tubes
Here is a reference for the using of the tubes as a ground tube. This is for CO Ground Storage Tubes but is applicable for any inert gas.
Here a company that leases and test said tubes
Here is a little information available at the CGA site.
Compressed Gas Association (CGA) Pamphlet C1, "Methods for
Hydrostatic testing of Compressed Gas Cylinders".
Compressed Gas Association (CGA) Pamphlet C5, "Cylinder Service Life-Seamless Steel High Pressure Cylinders".
Compressed Gas Association (CGA) Pamphlet C6, "Standards for Visual Inspection of Steel Compressed Gas Cylinders."
Compressed Gas Association (CGA) Pamphlet C8, "Standard for
Requalification of DOT-3HT, CTC-3HT and TC-3HTM Seamless Steel Cylinders"