William,
If you look really close you will see that there is only one RPO. Three Baggage Cars, One RPO, then two more Baggage Cars, then the Coaches.
This seems to be a some what usual arrangement of the San Juan Consist. The RPO "sandwiched" between baggage cars.
Why? Not a 100% sure, but I have heard when the Mail was extra large, or over sized, or for LCL Freight, they would add extra baggage cars in front of the RPO. We might have to get a historian to answer that one. The Baggage cars behind the RPO would be for baggage. Perhaps too many bags for just one Baggage.
Could also be an equipment transfer.
Many possibilities
Kevin Cook