At least, KP1858BM1 and KP1858BM3 are completely different designs, starting from the fact that BM1 is NMOS and BM3 is CMOS.
Also it is known that BM3 differs in execution of
HALT command: while the original Z80, whether it is NMOS or CMOS, makes idle M1 opcode fetches during
HALT (thus supporting DRAM refresh), BM3 simply ceases all activity. As the consequence, Z80 exits
HALT state with the granularity of 4 clocks, while BM3 exits immediately.
According to these two links: http://www.155la3.ru/t34vm1.htm and http://www.155la3.ru/k1858.htm, both T34BM1 and KP1858BM1 first were the clones of U880 design (or simply used the original dies of U880 from DDR) and then switched to differing (self-made?) design. U880 die has a clear writing of "U880/5" or "U880/6" on the edge, while the newer design has a heart symbol in the middle of the die.
According to this https://gist.github.com/drhelius/8497817, there are also some important differences in undocumented behavior between the original NMOS Z80 and T34BM1/KP1858BM1, while it seems that the authors did not try to distinguish U880 dies from later original KP1858BM1 dies, nor they specifically tested original U880s.