I wonder if the data plate was the only original information and the front left side frame stampings were done when the trucks went thru repaint, rebuild or some type of maintenance procedure where the body with data plate were separated from the frame. It would make sense to stamp the frame so you could make sure the right cab and frame find their way back together. If the truck in question never went thru this type of maintenance then that may be why you don't have the frame stamp. If it was me I would get a new data plate and take the numbers you can make out from the back of the original plate, use them and if they are short a digit I would add an X at the end. I know I will get plenty of people who will say well that's not how it's supposed to be done. This hobby is not an exact science and neither was the military's method if initially assigning serial numbers. Whatever numbers you pick wether it be the ones from the back of the plate or if you put any string of numerals you dream up, when you submit for the sf97 it will come back with the information you submitted. When you go to dmv with the sf97 to register it, the government has certified on the sf97 the year make model and serial number who is anyone to question it.