Also commonly referred to as Foot Rest or dog bone rest. Came in colors to match interior carpet. According to parts catalog was for cars through 1971.

Due to early cars having a fixed passenger seat (i.e., could not recline, angle or adjusted front to rear), the tethered footrest was included. The tether clipped to a D-ring below the speaker in the passenger footwell.
Click to view attachment
Click to view attachment
Click to view attachment
Click to view attachment