The grommet, p/n 352963-S, is documented on page 46 and 47 in the 1965 Ford Standard and Utility Parts Catalog (January 1965) published by Jim Osborn Reproductions (AM0202). From my research, the only Mustang application is 1964 thru 1967 "K" cars with manual chokes. I can't find choke cables listed in the Car Parts manuals, so it may not have been serviced.
Jim