Dust boot for the bottom of the gear shift. This fits early model ones (about 1973-75). The boot fits over a lip on the underside of the shifter extension rod around the hole where the bottom of the shifter protrudes through it. Later models do not have the lip and use a different design of boot that fits on the inside rather than the outside.
As there has been plenty of chopping and changing of parts over the years you will need to stick your head under the car and examine the underside of the shifter mechanism to see which type you have.