I've been using the K# that refers to the firmware used for a specific model.

For example:

K3 = Kindle Keyboard as it runs FW 3.x.x
K4 = Kindle non-touch from last year as it runs FW 4.x.x
K5 = Kindle Touch from last year as it runs FW 5.x.x
K6 = Kindle Fire from last year as it runs FW 6.x.x

Anyone's guess at this point as to the FW designations for all the new models.

This method is much more precise, especially since we now have a ship load of different models from the K1 through this years new entries.