Here is a summary of information available from Prof. Dr. Manfred Krebernik's article on the Fara texts (OBO 160/1 pp.275-283 etc.) which I used to pare down the list of unknown Fara signs.

The sign repertoire of Fara-period texts includes many simple and composite signs which were later abandoned, or which were replaced by other signs. Among these are:
LAK20 (probably a kind of sheep or goat)
LAK85 = GIRIx (SI x KID2)
LAK173 = KAD5 / ESIRx (see illustration)
ZU+ZU+SAR = LAK218, LUMx
LAK490 = GA'AR? (see Teuber 1995:26-30 for possible original form)
LAK647 = SIGx, later replaced by SIG5
LAK654, TA.gunu
LAK777 = UD / TUA?
EREN2+X
SHE+NAM2 = SHUSHx

Pairs of similar signs which later merged:
LAK6 (GIR2) with LAK7 (UL4)
LAK16 BAD with LAK17 TIL/USH2
the value IDIM might possibly be valid for LAK4
LAK23 SHIR with LAK24 GISHNU / NU11
LAK31 URI3 with LAK32 SHESH
LAK155 UMUM with LAK156 DE2 / SIMUG
ASH2 with ZIZ2 (not distinguished both under LAK162)
LAK248 KISH with LAK240 / 253 ANSHE / GIRI3
LAK359 HI / DUG3 with LAK809 SHAR2
LAK376 IM / TIM9 with LAK377 NI2 (later LAK377 survives)
LAK666 DUN3 / SUG5 (used for "Vermesser")
LAK667 gunu-form AGA3 / GIN2 used in the unit of weight GIN2
USHx vs. MUNSUB, both LAK672 not differentiated
GURUSH vs. KALAG / RIB both LAK 709 not distinguished
LAK775 LU vs. LAK780 DIB (LAK780 illustration not exact, cf. Ebla)
KU vs. DUR2 not distinguished under LAK 791
NAM2 vs. TUG2 LAK 792 not distinguished
SHE3 vs. ZI3 under LAK794 not distinguished

Not yet identified at time of LAK, but now identifiable are these:
LAK29 KAM4 / ZUBI
LAK159 NISAG
LAK173 KAD5
LAK226 SUMASH
LAK227 SUHUR
LAK296/7 ARKAB
LAK350 UZU
LAK384 ZAx
AK442 LAHSHU
LAK503 SHITA
LAK589 GISAL
LAK668 SHESH2 / ADKIN
LAK670 DARA4
LAK713 SHEN

Signs which were later split into a sequence of other signs:
LAK131 becomes UR3.BAR
LAK175 becomes IL2(.MA2).DUB2 and similar (SANGA2-6)
LAK212 becomes TU.GABA.LISH = ASAL2
LAK358 becomes EN2.E2
LAK742 becomes DUG.SILA3.BUR = BAHAR2