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3B59L104618

GM/Chevrolet called that a Vehicle Serial Number (a VIN was a later type of ID)

3B = 3200 (long bed 1/2 ton truck)
59 = 1959
L = assembly plant = Los Angeles
104618 = Unit Number = 104618-10001 = 103618th truck out of that plant that year

http://www.1954advance-design.com/chev-specs/1959-t-ser.pdf
Taken from the full 1959 Truck Specifications manual/document provided here (Information Kit) by the GM Heritage Center:
https://www.gmheritagecenter.com/gm-heritage-archive/vehicle-information-kits.html

That document is also on-line: http://chevy.oldcarmanualproject.com/chevyresto/59index.htm