Inside one of my NES Carts, I found a Famicom to NES Cart Converter!
Looks like the actual game itself was for a Famicom Cart, but was put inside with a converter by Nintendo for USA/Europe NES Systems.
Here's how you can find one. Go to any store that still sells old NES carts (Funcoland, GameStop). I forgot the games that did this, but look for ExciteBike, Hogan's Alley, Gyromite, or Wrecking Crew.
The cart should feel slightly heavier than a normal NES cart.
Look on the back to see if it has 5 screws. The cart can have 5 screws, but it does not guarrantee that it contains the converter. :smt045
On the front label, see if it does not have any grippers.
You want this:
Look on the inner edge where the slot chip is, facing the front of the cart upwards.
If the whole chip is gold, then you have yourself a cart with a converter.
If it has some green close to the inner part, then you have to try another cart.
The game should have a seal that looks similar in shape like this one:
But it should be in a black background instead of white & should write: THIS SEAL IS YOUR ASSURANCE THAT NINTENDO HAS APPROVED AND GUARANTEED THE QUALITY OF THIS PRODUCT.