I will disagree with Dan's statement that "The depressed-center flat is the transport carrier for the M-103 Heavy Tank...".
First, while there is a photo that appears to show an M-103 on such a car, there is some doubt that the car and load ever traveled more than a few feet. There is also doubt that these cars ever operated in the US.
Second. The shop manual for the M-103 shows a drawing of one loaded on the (prototype) non-depressed Roco car. I presume this drawing was presented as a typical loading.
I intend to place my (HO) M-103's on the plain vanilla Army 6-axle flats.