Ed Pawlowski wrote:

If, hypothetically speaking, a compressed-gas supplier handled
and processed ALL returned O2 cylinders the same way (both
welding and medical cylinders)

If it does not have that piece of paper, it is not medical oxygen.

I already replied to that. See the line above.
I'm working with facts, you are working with speculation and
hypothetical speaking.

You are not working with facts. You are working with pieces of paper.

It's ridiculous to say that two tanks that are handled identically
behind the scenes are nonetheless different because of a piece of paper.

But I'm not convinced that a medical tank has a lower
probability of containing a harmful impurity compared
to a welding tank.

You are welcome to your opinion. It may or may not be correct.
Find out for sure and be at ease with your speculation.

Since you claim to have actually filled commercial medical O2 tanks,
tell me what analytical testing you did to insure that their purity was
99.95%. What lab equipment did you use to determine the purity of the
O2 in those tanks?