STAM4NICK- BA boarding passes only ever show 1 of 5 letters:

M - Economy
W - World Traveller Plus
C - Club Europe/Domestic
J - Club World
F - First

Basically if you are booked in a discount or reward class your ticket coupon will show it but the boarding pass only ever shows the above- to avoid confusion at the gate (only one code for each class). In your case K is discounted economy and therefore your boarding pass showed M.

Anagnorisis was the victim of an unfortunate mistake on the part of an ill informed check in agent and should be compensated adequatley.

dnw