Is buttermilk a substitute for sour milk in a old banana bread recipe ?

Although, I will add that buttermilk is thicker than the "sour milk" that is called for here...only slightly, but you may need an extra TBSP

You can also add a tablespoon or so of vinegar to fresh milk and let it sit for 5 minutes before using.

Yes, it should work just fine. Have fun! ;o)