I can only say for sure that it changed between 2337A and 2419A!
But I can not narrow it further down.
My investigations so far are like this:
LCDType Bat.contact PCB confirmed Ser-Nr
LCDsepBoard tin/silver two separate 2326A,2337A
LCDfullMuPx spring/gold onePCB 2419A,2438A,2439A,2509A,2521A,2538A,2623B,2626A
LCDhalfMuPx spring/gold onePCB 3437S,3521S
The LCD changed to half as much Multiplex (MuPx) connections somewhere after 2626A and before 3437S.
These numbers are from my samples (which I can verify).
Most probably it depends on the manufacturing location and isn't consistent for different locations.
Hope that this fragment adds to the truth out there ;-)