Both the Mk1 & Mk2 could have a chrome or black locking petrol cap. These normal worked of a separate key from the ignition, boot & locks.
In case you want to swap locks about or key up an existing cap, removing the lock barrel is quite simple.
With the key in to lock to aid pulling the barrel out use a small diameter punch to push the pin out on the back of the cap. Hold on to the back and spring as the pin pops out and lay out to keep them in the right order. Then pull gently on the key to ease the barrel out.