There are three suppliers for Thinkpad keyboards: NMB, ALPS, and Chicony. You can identify which one you have (or which one you want to order) either visually or by the FRU number:

You can look under the keys of the keyboard at the pad/switch color and design to identify your keyboard manufacturer. This can be done by either removing the keyboard or using a paper clip to gently lift the bottom edge of the right arrow key.

You can look up your keyboard's FRU number using your 4-digit Machine Type (ie: 2081-CTO) and Serial Number here.

At this point, these distinctions apply only to the "traditional" Thinkpad keyboards, and not the "island-style" Thinkpad keyboards (Edge, X1x0e, X1)

Current Thinkpad Keyboards (enlarged Esc/Del keys)

Visual Key
Code:
NMB = red pads
ALPS = white pads
Chicony = grey pads
FRU Number Listings (columns, from left to right)
Code:
T420 - ALPS, Chicony, NMB
T420s - ALPS, Chicony, NMB
T520/W520 - ALPS, Chicony, NMB (*)
X220 - NMB, ALPS, Chicony
X220t - NMB, ALPS, Chicony

T410 - ALPS, Chicony, NMB
T410s/T400s - ALPS, Chicony, NMB
T510/W510 - ALPS, Chicony, NMB

Legacy Thinkpad Keyboards (normal half-size Esc/Del keys)

Visual Key
Code:
NMB = blue pads
ALPS = white switches, top right to bottom left (/)
Chicony = white switches, top left to bottom right (\)
For further help, see pictures at this Thinkpads Forum thread.

FRU Number Listings (columns, from left to right)
Code:
T400/R400 - Chicony, NMB, ALPS
T500/W500 - Chicony, NMB, ALPS
R500 - Chicony, NMB, ALPS
X201/X201s/X200/X200s - Chicony, NMB
X201t/X200t - Chicony, NMB
In earlier Thinkpads, Lenovo/IBM has labeled the columns on the Keyboard Service Parts pages, so I've left them out here. To find these listings, search:
HTML Code:
keyboard service parts <model name here>

A thank you to erik; without whom this page would not exist.