hi
I can only talk on the eley production , they do QC on all their products all the way through to test firing, the claim is some 600 checks, I have not seen all of these, some of their production can lead to poorer quality, this is their choice, if the product does not meet a criteria then the whole batch will be destroyed, so worn dies can still produce tenex, just as new ones can produce scrap, of what I know of RWS and Lapua they do similar types of QC just test firing changes slightly, just think of the variation in quality of lead, then the brass case and dimensions alone, Eley mix their own powder for rimfire, even more variation,
Batch numbers are just that a batch off the line are given the next number in sequence from the last one when it has been through the process of firing which finally gives it a brand eg match usually with the year in the number, the speed is also on the box of the top end of Eley and RWS not sure of Lapua, at the end of the day speed is not needed by the shooter, only does it group well, temp humidity and air pressure also play the part in groups on the day
steve