I Have A Question About My Pain Pills I Just

I have a question about my pain pills I just got some refilled and was looking at my last script compared to my last month script and in the description they have three different letter at the end.I was wonder what the different letters stand for.Seems like the script I just got filled not working as good as the month before.The script I just got filled is oxycodone 10 mg tab (KVK) and last month was oxycodone 10 mg tab (ZYD) Any idea what the 3 letters stand for.Thanks