Can someone identify this for me?

Can explain why it's not in the 72 wiring diagram.
Many people don't understand the importance of knowing the build date of their car. Chrysler quite often made changes during the model year of the vehicle.
The factory service manual is printed and distributed to the dealers before the model year begins, so any design changes after that will not be in the service manual.
This is an example of that situation.

In regard to the seat belt system, Chrysler made a mid- year change.
Up to Jan. 1972 the seat belt buzzer was not installed.
After Jan. 1972 the seat belt warning system was used on the abodies. These dates are approximations but they should be pretty close.

The change can be seen in the 1973 wiring diagram with the 3 dark blue wires coming out of the plug.
Now the parts catalog is updated 2 or 3 times during the model year, so the final revision was made in November 1972. In this final edition you will see the seat belt buzzer, Chrysler part #3620874, which was not in the first edition.
Of course all this depends on whether or not the previous owners made any changes to the car.