Late 2009, 27 inch iMac can use 32 GB of 1333 MHz RAM, beyond what Apple says - YouTube
Review: Apple's 27" big screen iMac (late 2009) | AppleInsider
Timetec 32GB KIT(4x8GB) Compatible for Apple DDR3L 1600MHz PC3L-12800 CL11 for iMac (Mid 2011, Late 2012, Late 2013, Late 2014 Retina 5K, Mid 2015 Retina 5K) SODIMM Memory Module MAC RAM Upgrade at Amazon.com
Apple Imac EMC-2308 | PDF | Macintosh | Airport
Memory Upgrades for Your 2009 - 2012 Mac Pro
How to Upgrade Memory in a Late 2009 21.5-inch or 27-inch iMac - YouTube