In the reare case you can not find a VIN or it is rusted away and your authorities give you a new number, how about ask them to allow you to stamp it into the frame on a easy to access area and get that noted into your documents. Asking sometimes helps, aspecially when these people have a heard for vintage vehicles.
To the experiance of this side of the Atlantic river, Kaiser Jeep products for the military relatively often have the problem of poor serial number stamping. Sometimes this is related to the number of depot maintenances, when the frame was sand blasted three or for times in the last 20 to 25 years in the depot.
I will not explain the "how to do" of this side of the Atlantic river for I know that our way of handeling the problem is illegal in the States.
Wolf