Is it possible the tanks you saw were not oxy-acetyline tanks?
40 years ago when I went through votech metal/welding classes, they told us to *never* lay down an acetyline tank with the regulator on it. We were told if any liquid acetyline flows into the regulator, then BOOM, off goes the tank like a rocket. I was also always under the impression that it is illegal to transport welding tanks without the safety caps on (and regulators removed obviously).
If you do have to lay down an acetyline tank, with the regulator off, and safety cap on, you were supposed to roll it around on the ground first while it was still on it's side, then stand it up and let it sit over night before you took the cap off and put the regulator back on it.
Even if they were "empty", I wouldn't lay down an acetyline tank, even with the safety cap on.