A couple of things here (as the OP).
20# tanks do not fit my double cylinder bin. They have to be up on spacers and they bang around. So much for exchange.
I am not talking about a visual recert while getting a refill. That is good for five years and I wouldn't trust it myself.
I'm talking about a hydro-tank pressurization test (the same as used on SCUBA tanks, which I also have and use). The tank is submerged in a water vat (to absorb the blast if it explodes) and pressurized to 2.5 times the working pressure. The date and location of the test is then stamped on the collar. This is also the test for MIG welding cylinders (which I also have and use).
A
(ASME tanks, including those in most Class As, are exempt.)