Advance Design trucks, mid-47 to mid-55, did not have a factory serial number stamped on the frame. There should be a plate in the driver's side door post, if that is missing you don't have a serial number.
In that era engine numbers were often used on titles.
Laws regarding titles vary from state to state and I've no idea what they are like in Missisippi, but in Washington it is a felony to move an id plate from one vehicle to another.
