Pellets with the same die number do change as I have some Air Arms die 2's from 9/14 and 2's from 11/14 and they are both 4.52 and very good, but slightly different.
I have some older AA's with the highest die number being 31's from 2012.
Still using die 9's from 26/1/15 and I bought all of them that they could find in Redbecks.
The best I ever had were AA's from 2007 with a 00 die number. Work that out.
Also, I think that anyone that has measured them knows, that if you buy a tin of 4.51 or 4.52 it doesn't mean anything other than they are .177, I purely go on die number and date now.