If you consider Pilgrimmage, I don't understand why you think Mint is different. Each one lets's you select a card from a certain zone and allows you to gain a copy of it, which may not be possible. What is the difference between buying Pilgrimmage with Diadem and any number of Heirlooms in play, or playing Mint and those same Diadem and Heirlooms in hand? The 3 cards on Pilgrimmage can't really be the difference, because it doesn't add anything, just makes the effect 3 times as big.
Excellent point.
I was vaguely trying to identify cards that let you gain a card of choice from Supply but where you can actually choose a card not in Supply - something that can create confusion and therefore warrants further explanation.
I realize that Smugglers and Changeling are the only cards that qualify. The reason is that they say "gain a copy..." as their first instruction (and "gain a copy" = "gain a copy from supply"). Pilgrimage is different, since the first instruction is "choose cards you have in play". Mint too, since its first instruction is "reveal a card from your hand".
Technically of course, all these cards function the same way: first you choose from a set of cards that is not Supply, then you gain a copy of your choice from the Supply. Smugglers and Changeling are different because the initial choice is implied. Workshop and friends also have this initial implied choice, but the choice is among cards in Supply.
Here, then, is the conclusion: Smugglers and Changeling uniquely give you an unstated choice from a set of cards that is not Supply.
gain a card costing $x = choose a card from Supply costing $x, and gain it from Supply
gain a copy of a card from set x = choose a card from set x, and gain a copy of it from Supply