What markings identify an "exchangeable" welding gas cylinder? I'm
shopping casually for a used oxy-acetylene rig and am having
trouble determining what my local Airgas dealer will exchange.

Decades ago I had an O/A rig with owner cylinders, but all my
exchanges were at the same vendor (VB Anderson, if it matters)
so there was never an issue. I don't even remember how or if the
cylinders were marked. Now the nearest major vendor is Airgas and
they're rather unclear about what they will and won't exchange.

I do recall from recent experience with semiconductor gas cylinders
that some are "prepaid demurrage", which isn't refundable and the
cylinders can only be exchanged at the original vendor. Semiconductor
gases are expensive enough that the cylinder does not matter much,
but it'd be an expensive mistake for a welding gas if that practice is used.

Thanks for reading, any counsel appreciated.

bob prohaska