The parts manuals list multiple part numbers for serial number effectivity, there is usually a different letter for a particular serial number identifying the part. The easiest way that I have found to get the correct P/N is to use the wiring schematic for your serial number, as all of the components are listed by P/N in the equipment table of the schematic.

Steve,
All those are superseded as follows:

1579-2 is 111-140D
S1660-1 is 8781-10
S1991-1 is 111-138D

You can purchase all from http://www.cessnapartsdealer.com/ (be advised, their site is down at the present time for maintenance)

You may also purchase from Lane Aviation at http://www.laneaviation.com/estore/