It took quite a while, but I finally was able to get the PRR FM decals from John Frantz of Mount Vernon Shops. He made these S-scale decals on special-order, and was able to sell all that he had made in very short order. If you are looking for these decals, you can contact John (he takes PayPal).
The photo shown below is after I applied the decals on one side. John's decals match the lettering shown in the Pennsylvania Railroad Flat Cars book, with the only exception of the letters "PRR" missing from the text next to the class name "FM" (see bottom of page 25 of that book). Not yet updated on the subject of the photo, I found some leftover decals that contained small "PRR" letters, and used those to complete the model. The decals are of high quality and I had no problems applying them with filtered water and Walthers' Solvaset.
Another one of the cars that has the decals applied. You can see the "PRR" next to the "FM" in this photo. After the decals dried overnight, I gave them a light spray of Testors Dullcote.
Next, I took the car to the garage and applied a light coat of the original body color with the airbrush. This cut down on the bright, new look of the white decals. Further weathering will continue to tone those down a bit.
And here is a family shot of all three of the flat cars I built as part of this project. Other than for some weathering, and possibly the replacement of the trucks and the installation of the Sergent Engineering couplers, the cars are finished.
I grabbed a few loads I had available and put them on the cars. Of course, the actual loads will need to have proper tie-downs.