For iMac (late 2015/Retina 5K)  
iMac"Core i5"3.2 27-Inch Aluminum - MK462LL/A - iMac17,1 - A1419
iMac"Core i5"3.2 27-Inch Aluminum - MK472LL/A - iMac171 - A1419
iMac"Core i5"3.3 27-Inch Aluminum - MK482LL/A - iMac171 - A1419
iMac"Core i7"4.0 27-Inch Aluminum - BTO/CTO iMac171 - A1419
  
General tech specs  
Technology DDR3L
Density 8GB
Module type SODIMM
DIMM type Unbuffered
Voltage 1.35V
Module quantity  2
Number of Pins
 
 204
Total Capacity
 
  16GB
Speed & timing  
Speed DDR3-1866 MHZ
CAS latency  11
Package List 2Pcs Crucial  8GB DDR3L 1866MHz SODIMM RAM