There were two different gas (petrol) caps used. One was a flush fitting screw in cap which was painted body colour. The other was a chrome flip open race style cap with the word "Fuel" on it.
Flip open options (the secondary cap)
a. cap, non locking
b. locking cap
In the day starting in 1969 many things were happening and people started to lock their homes and cars up at night, hence the locking gas cap option (this is indicated as J46 on 1970-71 broadcast sheets).
However Jim's Auto does sale an option to make the flip top cap a one pieces open cap. see link
http://www.jimsautoparts.com/fuelpart.htmPN# FW-671
Hope this helps