As Bob stated, the Ford nomenclature is "insulator - front bumper arm"
By the time that the 1975 "final" version of the 1965-1972 MPC was published, there were no 17B947 parts serviced for any year Mustang.
I found the diagram where the 67-68 insulator is shown, but for the 69-70 (see P-6664 attached) this part is not called out.