Back in the 70's and 80's, when you purchased most pickup trucks they did not come standard with a rear bumper. So many folks had special needs for the bumper type (towing, campers, etc.) that the manufacturers made it an option. You could order it from the factory, but it was not mandatory.
A little trivia - when I ordered a Ford Econoline van in 1977 I found that almost nothing was considered standard. If you bought the basic truck it had only the powertrain, uninsulated metal box with roll down front windows, and a driver's seat. I had to order the passenger seat, side windows in the door, back windows, spare tire, rear bumper, radio, etc. The standard powertrain was a 6 cylinder with 3 speed manual on the column, no PS, no PB.
Bruce