Full Details from Amazon.com : http://www.amazon.com/gp/product/B009V116L6?tag=onigi-20

8GB Memory RAM for Lenovo ThinkPad T Series T420s 204pin PC3-10600 1333MHz DDR3 SO-DIMM Black

Brand new certified 204pin PC3-10600 1333MHz DDR3 SO-DIMM Black Diamond Memory
Low density 8GB memory single stick memory module
100 compatible garrantee for Lenovo ThinkPad T Series T420s
100 pre-tested on Lenovo system to ensure functionality
Lifetime warranty