I guess the first question when considering a shielding can (before 'when might they have existed') was how on earth would they have got them in there in the first place.......given the ES has no cavity access rout.
Another question to have thought about was if this supposed can is around the pots, and the pots are spanning the f hole, wasn't there an easy way to look into your guitar and see whether or not your guitar had such an item very easily visible without spending a whole heap of time posting to various forums.