Chevy did offer a dealer installed "locking gas filler cap" in 68, #986970. But it was a standard cap and not marked SS.
In 68 I remember buying gas as low as 19 cents, so I doubt if there was much of a market for original lockable gas caps. Never seen anyone making one today... But I don't doubt if the market might urge someone to design one. The original owner in the Miami area of my 68 coupe did buy a lockable cap.
As to fit, you're right. But the day of sealing caps had not yet arrived and did you ever try to put gas in the back fin filler of a 57?