Sales order with Supplementary items
Hy,
I think you need three items, cylinder (empty), gas and the finished product cylinder with gas.
So you sell cylinder incl. gas, you can produce cylinder + gas using with production order or even report as finished for each filled cylinder with the gas.
You can use return order for the empty cylinder you get back from customer.
I would not use supplementary items in this case.
Hope thsi helps.