I've just realised that Maistres isn't Masters, that should be Maîtres, so are the spellings archaic or is there some poetic licence? My French cousin doesn't recognise the work Estainier at all. (There is an old profession called Painter-Stainers in the UK, no idea what the French equivalent would be though, and I'm not even sure if there's a connection!)
Pewter doesn't seem to make any sense in respect of this though. The label says Estainiers not Estanniers, and my dictionary give étain for pewter, not estain, so I'm still puzzled.