They are most likely referencing “date codes” Date codes will be a two letter code, associated with numbers.
A =0 ; B=1 ; C=2 ; D=3 ; E=4 ; F=5 ; G=6 ; H=7 ; I=8 ; K=9
For example; A gun made in 2009 would be marked AK (for 09) or guns produced this year would be BK - 19
The date codes can be found on the slide. I’m not sure about USPs, but I know the Mk23s with date codes KG through KI are more desirable. The ones in the late nineties had a more durable maritime finish.