Idler Pulley between Water Pump and Crank Pulley?

[JGC403]

I'm finishing up the BB in my '85 Ram, I am collecting all of the pulleys that I need, from my grandfathers garage, there are a bunch of different pulleys up there from small blocks to big blocks. Anyways we have a 1965 Chrysler New Yorker with a 413 I was going to use this as a reference to figure out the pulleys and brackets that I need. While looking around in the engine compartment, I noticed that there is an idler pulley on the belt for the water pump to crankshaft pulley.

From what I have found is that AC cars only had this idler pulley. Why is that?

 

[Jim Lusk]

AC cars used a separate belt for the water pump and only the water pump, so an idler was necessary. On AC cars the AC compressor and alternator were run by a pair of belts.