eip?

[bonesoul]

does erc721x even have a submitted eip?

[sendrosa]

This implementation is essentially the ERC1155 (EIP1155) with backward compatibility. Also, when ERC721x was created, ERC1155 wasn't supported by Metamask (or another wallet) and overcame the difficulties with this implementation.

In conclusion, the description of the functions is described in EIP1155 and has been implemented in a different way to serve the needs that existed.