the Mother in Law seat, there were a couple versions, but they mostly were exposed to the weather and not equipped to keep someone seated in an emergency braking event

There was this variety that slid out from the side, and I've seen it on the Kissel and and the Paige Model 6-55 Daytona Speedster

and there was this flip lid on this 1911 Olds Autocrat

https://justacarguy.blogspot.com/2017/08/1911-olds-autocrat-has-cool-rumble-seat.html

the 1910 Matheson had one mounted to the spare tire /5th wheel

Paige Model 6-55 Daytona Speedster with mother in law seat

and the 1920 Kissel Gold Bug I posted back in 2013

https://justacarguy.blogspot.com/2013/04/1920-kissel-gold-bug-roadster.html

https://fornoob.com/the-mother-in-law-seat/