Earl,
Santa Fe rider combines 2602-2608 were rebuilt from baggage-buffet-lounge cars 1342-1347 (built 1923) between 1936 and 1940.
Rider combines 2609-2611 were rebuilt from coaches in 1949. These were from the 900-series cars built in 1913. Photos in the Santa Fe head-end car book shows that these had the forward vestibule steps removed and the side doorways plated over.
Need to look at the book by McCall about the mixed train combines. I seem to recall that some were rebuilt from early steel straight coaches to replace wooden cars -- some of these might have kept both vestibules.
Brian Norden