Can anyone tell me how the locking gas cap on an original Mercedes gas cap works? Mine is stuck, and I cannot seem to unscrew it. It spins, and it feels as though there is tension behind it. How can i get it off? Thank you.

1967 Mercedes 250S

 

A quick note to let you all know.......I got the gas cap off. In addition, I fixed the ignition with a 'hot spark' no more points! yeehaaww