belt sizes
 

I lost the a/c belt on my '81 300sd. Called the dealer to get the right belt. They list 4 sizes for the '81 300sd! I gave them the vin and they said that it is a 5x925. Fast Lane lists it as a 13x925. Phil is not around today and I need to get this done. Are these metric sizes? Which one is correct? Is there a way to convert it to a number that the Auto Zone guy will be able to use? Thanks!

Dann