The sub-circuit board, consisting of five semi conductors and additional passive components, was introduced in 2002 with circuit #04. Circuit #03 which was superseded in 2002, still had one discrete FET only.
As to significant changes of the U87Ai from versions #1 through #3:
only minor changes/improvements were introduced, which do not affect the product audibly in any way: anything from better circuit tracing material, to changes in switch material to stamping the serial number into a different part of the bottom bell. The big audible change came with the introduction of the daughter board. The mic equipped with circuit 04 sounds a bit more fuzzy and unengaging.
2015 brought circuit version 06 to the U.S. Lowest serial number I recorded for 06 versions is 123xxx. Now we are back where we started: the good old 2N3819 FET, this time the Fairchild version, is once again individually biased, (it is also used in the new U47fet reissue) and the daughter board is gone. Big improvement.
I have recently posted about the K870 capsule quality issues in another thread.