Most likely you could bet that 95% (99%?) of original 1951 1/2 ton trucks came with 16" wheels. There was, however, a Regular Production Option for 15" wheels.
Also, not to be argumentative, but saying that a 50 year old truck is all original (even if it has been in the same family for 55 years), is like parents saying their beautiful 20 year old daughter has never been kissed - it's possible.
