They probably got a SMB1 cart, flashed a hacked rom onto it, and put on a custom label. Of course, if it works and plays on an NES, it could be fun to have, just don't pay a fortune for it.
There's an entire site devoted to this kind of thing - flashing roms (or translations) onto carts so you can play them on the native system (the original FF3j comes to mind). Not much harm in making bootlegs of 20 year old games that were never released in America, I guess, the site I saw didn't want more than $20-30 for a game, which includes expenses and shipping.
Note: I know CAG policy on bootlegs and I'm not promoting this kind of thing, please don't ask me for the link.