Regarding Espee Observation cars; most were built by Pullman with stainless steel siding over the CorTen sides. This was somewhat of a waste of stainless, as the cars were also painted!!! However, this construction technique caused the car sides to rot out from water that would leak in,( usually thru the window gaskets), and slosh around between the stainless outer panels and the CorTen inner panels & frame members. This " Pullman Disease " affected all Espee cars from the original Coast & San Joaquin Daylight consists, and others as well. In the 1950's Espee stripped the stainless off & rebuilt the sides on many cars; perhaps this was when the Obs' were converted to all coach seating.
Regarding "... the Obs used on the excursions," if you are referring to the 1984 World's Fair Daylight, the Observation used was an ex - Great Northern car, the Appukuny Mountain.