I will concede that spare cylinders were never carried in the American West, if someone can give a logical explanation on why loaded cylinders have turned u in excavations and building renovations in the American West.
Its called out of context. We do know there were a lot of conversion cylinders procured to replace the BP cylinders.
So, for each Conversion, you would created one excess BP cylinder.
So unless you find a cylinder on a layer that also has a carry pouch with it, context is not there. All you can say is you found a BP cylinder.