Tired of the awful OEM screen on your T420? Want FHD (1920×1080) resolution with the full colors and deep blacks of IPS screens? Having a hard time deciding which to order? Then this page might help you.
There are several screens available for the non-S T420 laptop which have performance characteristics on par with true IPS screens. This page has a good explanation of how the various panel types differ.
This thread by RMSMajestic seems to be the origin of the screen upgrade for the non-S Thinkpad T420 and T430. The first post explains why flickering is a common problem.
There are several eBay sellers carrying FHD IPS controllers. The one sold by moe-tech, for example, supports the following screens:
1920×1080 (FHD) Screens
|B140HAN01.1||AHVA||Matte, hard||16.2m||300 cd/m^2||700:1||89/89/89/89|
|B140HAN01.2||AHVA||Matte, hard||16.2m||300 cd/m^2||700:1||89/89/89/89|
|B140HAN01.3||AHVA||Matte, hard||16.2m||300 cd/m^2||700:1||89/89/89/89|
|LP140WF1-SPK1||AH-IPS||Matte, hard||262k||300 cd/m^2||700:1||80/80/80/80|
|NV140fhm-n41||ADS||Matte, hard||262k||250 cd/m^2||800:1||85/85/85/85|
There are also 40-pin controllers that support 2560×1440, such as this one by e-qstore. It lists the following screens:
2560×1400 (WQHD) Screens
|AUO||B140QAN01.1||AHVA||Matte, hard||16.7m||340 cd/m^2||700:1||85/85/85/85|
I chose the AUO B140HAN01.3 from eBay seller neidersch, and the LCD controller from moe-tech. Why that particular screen? Because I wanted a matte finish, and it appears to be identical to the B140HAN01.1, aside from being 0.2mm shallower. Both have been working great for around nine months.