The bottles empty weight is give the name "Tare".
Calor mark this weight by punching figures into an aluminium disk, a collar trapped between the bottle and its valve assembly that is screwed into the bottle.
In Calor's case they, for reasons that escape me completely, give in in imperial pounds and ounces????
Some more enlightened other LPG vendors stamp the "Tare" weight, quoted in kgs, of the bottle, into the steel ring welded on the bottom to create a base.
Edit: and as quoted in #4, Flo-gas, with its steel bottles and some of its satellites simply stencil paint the bottle's Tare, in kgs, on the shoulder of the bottle; IMO nice and convenient, easily seen and not requiring conversion from an imperial value.
Flo-gas with their Plastic composite bottles, "Gaslights" have in moulded into the top edge of the plastic, valve protection collar