The P38 had a 'dickie' seat in the boot, on the optioon list; it was originally made for Overfinch for thier coach-built Classics, and you could get an aftermarket kit to retro-fit them to the Classic or P38.
Not much good for any-one over about nine years old though.
Had one in my old Rangie.
Rare as rocking horse do-da second hand, but I believe some-one is still making them to order if you hunt hard enough; BUT, big issue with insurance companies..... they dont like added risk of extra seats!