It has been my dream for at least four years now for the WW&F to rebuild a replica Coach #6, which was the Mail/Baggage/combine car that you are talking about. The reason for that is because somebody once talked about building a straight RPO car, to show the general public what a RPO would have looked like. So I came up with the idea of Coach 6 because it can hold around 12 passengers or so, you can put a wheel chair person with their family, and also the general public can see the mail clerks station, and what it would have looked like. The best part yet is that the car was only around 35' long, not the average 45' long coach length.
I know this isn't the information you wanted, but I look forward to seeing your plan once you have it completed. Do you know about the plans in the Two Feet to Tidewater book, and also another book as well, I forget which one, also has the plans for Coach 6 in the back of the book. Maybe this could be of some help.