First, 1953 panel is correct.
The official names of GM items installed on new vehicles delivered by GM to the dealer were:
Standard Equipment - the base equipment as described in Truck Data Books (at least into the early 60s)
Optional Equipment - factory Regular Production Options (RPO) - this were add-on or optional equipment ordered by the dealer and installed at the factory.
Accessory Equipment - GM Truck Accessories were ordered by the dealer or by the owner - these accessories were installed by the dealer, by the owner, or by anyone.
Special Orders (from the factory) - usually these were paint options for colors/combinations not listed as RPOs
Special Equipment and/or Vocational Equipment (sometimes described in the Truck Data Book and listed in the Silver Book for that year) - special bodies and special equipment, such as dump bed, ambulance, school buses, specially-equipped step-vans, "woody" bodies, etc. - the dealer would take the order and the factory and special-equipment company coordinated the transportation/delivery.
In 1953 and 1954, Chevrolet stated that "Approximately 65% of all Chevrolet trucks sold require some alteration or the addition of some item of vocational equipment to meet the user's requirements."
And, the dealer might install any GM or other after-market parts/accessories that suited the dealer's and/or the owner's fancy.
