To add to Brad Allen & Tiny

1948 - 1951 Chevrolet Truck Shop Manual
UNIT SERIAL NUMBERS Page 1...Page 2
VSN = Vehicle Serial Number
VIN = Vehicle Identification Number
ESN = Engine Serial Number

They called it VSN back in the day then started calling it VIN later, can't tell you when the change happened.

GM Vehicle Information Kit 1949 Chevrolet Truck

1954 Chevrolet 3105 Deluxe Panel Truck
w/Factory HYDRA-MATIC