Today I was able to replace a ThinkPad T420 keyboard with the one from the newer model T430.
Actually, it's quite common to see people doing the opposite: replacing the T430 keyboard with the T420's. The new ThinkPad 25 brings back this keyboard. But it's not a matter of preference, but of price.
This keyboard as a replacement part seems to became very rare to find with a reasonable price. A used one costs around U$120 and U$180 (Brazil). Just the keyboard. A used T420 costs around U$300. The whole computer.
I was able to find a new, T430 keyboard for around U$15, shipment included. At a reddit forum, I saw it was possible to install on T420 because of a customer who had the keyboard wrongly replaced by Lenovo.
With a cut here and a double-sided tape there, a few adjustments on .Xmodmap, we are set.
! ThinkPad T430
keysym XF86Back = Page_Up
keysym XF86Forward = Page_Down
keysym Next = Insert
keysym Prior = Delete
keysym Delete = Home
The backlit won't work, since there is no way to connect the power supply in it.