Mark,
The image was scanned from the 1949 Chevrolet Truck Data Book for Chevrolet Salesmen. A Truck Data Book Supplement was provided in 1950 (with a booklet describing differences between 1949 and 1950 trucks).
Jim Carter sells a 1949 reprint. The 1950 supplement is most likely not available in reprint.
GM also published those books in A-D years for 1947, 1951, 1953, 1954, 1955-1st, (and later years, and earlier years). I am not sure if there were separate books published in 1948, 1950, and 1952. If someone has one, maybe they will post a confirmation.
Those books have the most extensive information about what was standard and optional for each year. If the price list is included, all accessories were also listed. These books and GM Technical Specifications Manuals (available from the GM Heritage Center) are the best reference information for helping people find out what was
most likely original equipment and original options/accessories.
Most likely because there might be errors, omissions, or "holdovers" or "leftovers" parts at the beginning or end of any model year (and, dealer-installed oddities).
