Would like to purchase a fixed mast antenna

Originally AM-radio-equipped cars would've had a 76.5" telescoping mast while FM had a fixed 31" mast to maximize FM reception. Both are shown in the 1972 parts book.

You only need to replace the mast; the base is the same between the two. It was common for dealers to replace telescoping masts damaged by garage doors, car washes, etc. with the stronger solid FM mast.