Dave:
I just got back home from Colorado and New Mexico.
I'll be posting some info on this topic in the next day or so.
Your guess would have been as good as any until Erick and I checked out all the cars in Chama and most in Durango after I posted the above message.
The additional wood is not to support the floor, as the pieces are only 8" high, and fall about 4" short of the top of the long sills.
I'll post the answer tomorrow, if I have time.
Mike