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... (More)