Unicomp was still making keyboards for IBM as late as 2004. P/N 18P7970 (a Model M13) is an example of one of these very late IBM-branded Model Ms.
Typically, ISO layout Model M keycaps do not have the word description printed on shift, backspace and enter keycaps. Also, ISO caps locks typically have "Caps Lock" spread across two lines instead of all text on one.
The tooling for the Model M15 (IBM Adjustable Keyboard and Lexmark Select-Ease Keyboard) was sold to Maxi Switch by Lexmark in 1996. It seems Maxi Switch did nothing with the tooling.
It is possible to greatly enhance the performance of the trackballs of Model M5-1s and M5-2s, TrackPoint IIs of Model M4-1s and M13s (or even old ThinkPads), and the pointing sticks of Unicomp EnduraPros by using InterAccel by KovaaK. It's designed to grant QuakeLive-style mouse acceleration across all of Windows and it works wonders with these older pointing device technologies.
You can order the old-style orange APL symbols keycaps from Unicomp by ordering a Custom Printed Buckling Spring Button Set with "101 UNOM5623 on Pearl/Pebble" (without Windows keys) or "104 UNOM5623 on Pearl/Pebble" (with Windows keys) as the comment.
Unicomp's internal codename for the Model M4/M4-1 keyboards (marketed as the Unicomp Might Mouse) was "Surf".
The sale of IBM Information Products Corporation and the formation of Lexmark did not mean the end of IBM-produced Model Ms between 1991 and 1996. IBM UK (Greenock), Mexico (Guadalajara), Canada, and some other US plant continued their own production in some capacity after Lexington and the 'other' part of Greenock fell to Lexmark.
Unicomp once combined the Model M5-2 and M13 designs together to create a keyboard that has both a 25mm trackball module and a Unicomp pointing stick. It was known as the Unicomp On-The-Ball Plus and Unicomp Trackball Plus Keyboard. P/N UNI0496 is an example of one of these.