The Displaytech monochrome LCD part numbers follow a standard system. The first set of numbers is for the product series:

Character Modules
The numbers stand for the number of characters 
(e.g. 161 = 16 characters x 1 line; 204 = 20 characters x 4 lines) 

Graphic Modules
The numbers stand for the display resolution (e.g. 64128 = 64x128 dots; 128240 = 128x240 dots). The letter stands for the version (e.g. both 64128K & 64128M are with the same resolution, but the size of them are different, so they are with different letter as identification)

The remaining details for the other items can be seen in this document.