The Roco flat most assuredly did have a prototype. There were quite a few in Germany but probably not in Austria after the occupation ended.
I have created a photo album named "army flat car" to which I have uploaded two photos and the applicable (I believe) page from the TM.
The two photos show flat cars (not depressed center) with the same or similar trucks to the car in the photo that started this thread.
As can be seen (I hope) on the TM page, there were depressed center flats in that group with Buckeye trucks and others with the as yet unidentified truck.