ABO is tested for moisture on an individual cylinder basis. Medical O2 is tested for purity on a sample basis (usually one cylinder out of a 20 cylinder line). Industrial O2: each cylinder is sniff tested prior to filling to make sure no flammable gas or other stinky thing is loitering in the cylinder. No testing of industrial grade after filling.

Yes, all are filled from the same liquid O2 tank, all are pumped on the same manifold. However, the compliant filling facility does the above steps because either FAA, FDA, or CGA says that is how to do it.

It's reasonably easy to play safe: have a CGA 540 tank, get a fill from an ABO cascade manifold at the airport. It is not reasonably easy to get a "great price", especially if you fly where there is no ABO manifold available..