RAM & Memory Upgrades
OWC - 16GB OWC Memory Upgrade Kit - 4 x 4GB PC10600 DDR3 1333MHz SO-DIMMs for Mid 2010/2011 21.5" & 27" iMac models
16GB OWC Quad Channel DDR3 SO-DIMM Kit (4x 4GB) 1333 MHz Clock Speed - PC3-10600 1.5 V, 204 pins, non-ECC RoHS Compliant 9-9-9-24 Timings
https://www.bizrate.co.uk/ram_memoryupgrades/owc-16gb-owc-memory-upgrade-oid6712534709.html