Well I have quite a long post for today but I hope everyone will enjoy it. While working on an effort to inventory some of my collection, I have been going through some storage boxes that have sat unlooked at since we moved to Texas some 15 years ago. What I found, according to a note I left in the box, was something that I purchased when I was first starting to collect in Dayton, Ohio from a train show in the late 70's. So this item has been moved around the country waiting to be "discovered" again for some 45-50 years!! Anyway I hope you will enjoy this.
This is the outer or set box...it never had a locomotive or tender while I had it, likely missing from the set or perhaps it was not part of the set when purchased new. Note that the label reads "A.C.Gilbert Company, New Haven, Conn" so it post dates Gilbert's acquisition of American Flyer in February 1938 and the move of train production from Chicago to New Haven.

Inside the outer carton, are the component boxes. Still nearly fully intact they contain the individual cars that made up the set. These cartons bear the notation..." B750 Copyright 1938 by the A.C. Gilbert Co. A..C. Gilbert, New Haven, Conn"

The front of the component box.

The side of the component box

The ends of the component boxes, clearly stamped "3171 Pullman" and "3172 Obs". Note the "R" designation on the box. IAW Greenberg's Guide to "American Flyer Prewar O gauge" by Alan R. Schuweiler, Kalmbach Publishing Co, 1997 these 8 1/2 inch cars were offered 1930-33; 1934;1936-38 in various colors and configurations and this configuration, which you shall see, which has the Type XII trucks and Type X coupler was listed as #3171R in the catalog but not on the car (which in fact carries no number).

Note by an unknown individual on the bottom of one of the component boxes, perhaps by the original dealer, maybe relating to the "new for 1938" Type X coupler but this reads (written in cursive, with a pencil) ... "Special Coupler".

Here are the cars, # 3171 Pullmans and a #3172 Observation

A close up of the Pullman. Identifying and dating information for both the Pullman and the Observation are Color (Red over Red), Wording of the Name plates. (American Flyer Lines), position of the nameplates (over the windows and under the windows to the left and right), Type XII trucks and Type X couplers. This combination of characteristics dates these cars uniquely from 1938.

The matching observation

Here is something I find rather amazing. These cars are all illuminated, taking power individually from the track. But look at the undersides of the cars, they appear almost unused. There is some slight wear on the one pick up but there is not obvious wear any where else on the underside.

Well that was a long story and I hope you enjoyed it. Now I have to find out what locomotive pulled this set and then find that!! Likely a long process 
Best wishes for a great weekend
Don