Can be used on many REAR inner side fenders  as shown... as well as trunk floors... just measure yours.   Size to fit a 23mm hold.. or about 15/16th.

This plug's for most and many of the 70-81 era GM Cars particularly Firebird TA & Camaro

This metal plug has the 2 small ears under it to press up against the floor to lock them in.

(NOTE.. Thanks to Ben N. for the photo of this in place)