Some are aware of the Caltron 6 in 1 NES cartridge, but there was also plans for Caltron 9 in 1. The 6 in 1 was released, but 9 in 1 never released due to Caltron being in a lawsuit with Nintendo and ultimately sold their business. However, this doesn’t mean that a copy doesn’t exist.
An NES prototype of 9 in 1 has surfaced on the online auction site eBay. It’s believed to be the only known prototype in existence.
It’s also worth mentioning that the game was verified by VGA as being real with a certificate of authenticity. The seller, leathal80, has a feedback score of 300 with 100% positive feedback. The current asking price is $5,900.