Presume that the bottle has a serial number and the missing receipt would have that on it. With the aid of a name and postcode SGS should be able to match them together.
i have several of their bottles with the same 'deposit paid 2014' that i aquired whilst helping a mate out... i was working on the principle that i could swap them out, but probably not get the deposit back if that makes sense? No idea if thats correct or not though!
ime sure ime using sgs gas and i just take them in for a refill..ive never had to show a recipt and 2 of the ones i have argot and a mix bottle are ones ive bought with welders ect and got them swapped out fine..
ime sure they want their empty bottles back as much as you want a new one