Famicom Mahjong controller and how does it work?

This game called IDE YOSUKE JISSEN MAHJONG 2 comes with custom controller

https://www.ebay.com.au/itm/-/173632428 ... _cvip=true

I've been wondering how does this custom mahjong famicom controller work?
I've thought that NES/Famicom will not support any extra buttons aside from the standard buttons on standard controller?
Does it means that one could develop a game with custom controllers that provide more buttons than standard controllers?

Yes, you can read more than 8 buttons. You can use an SNES controller easily, the only difference is the connector. Spookotron is one NES game that uses an SNES controller. I built the adapters that shipped with the game.

I'm not sure what's inside that Mahjong controller, but most likely it's the usual shift register chip used in NES controllers, and you can simply chain several of them together.

Thwaite for NES can read a Super NES Mouse through the same adapter, and the Mouse's report is 32 bits long.

We've documented this controller here: nesdevwiki:Jissen_Mahjong_controller