That definitely is a period-correct spring-mast aftermarket antenna. People who drove where there were lots of low-hanging branches tended to like those antennas as the normal masts would break off or get bent. That particular style of antenna fixed that.
There were some replacement masts for the screw-in masts which were that style, too, but a little pricey compared to the normal 3-section mast antenna. The 31" stainless steel mast was the best way to go. There were also some 3-section masts which were teardrop shaped to decrease wind whistle by the mast, usually on GM cars of that time.
There was a comment in "Motor Trend" (or similar) back when the stainless steel masts came out that some teen-aged males in some northern cities would walk down the sidewalk and karate chop the normal mast antennas, breaking them off. Allegedly, when they tried that on a 31" stainless steel antenna mast the first time, it came back "to bite them" rather than breaking off. FWIW.
The 31" length was supposed to be optimum for FM radio reception, which was becoming a popular option (even stereo multiplex) in those later 1960s times. Best way to ensure the customer got good reception was to optimize the antenna length, non-adjustable.
Just some recollections,