Numbers are 0 to 9.
The numbers in the grey cells repeat horizontally and vertically and are present in every nonet.
No repeats in any other rows or columns.
Each of the other numbers is missing from one and only one nonet.
Five pairs of unknown numbers are semi-symmetric, with the corresponding cell containing the same number or its partner.
Note that in the steps below locked, placements and killer pairs in R6, R7 and C6 don’t apply for R67C6, as appropriate. However placements and locked for N3 and N6 DO apply for R67C6, as appropriate.
15(3) cage at R7C1 and 15(2) cage at R7C3 overlap at R8C3
Prelims
a) 14(2) cage at R2C5 = {59/68}
b) 4(2) cage at R2C7 = {04/13}
c) 14(2) cage at R4C5 = {59/68}
d) 4(2) cage at R4C7 = {04/13}
e) 11(2) cage at R4C8 = {29/38/47/56}
f) 5(2) cage at R5C8 = {05/14/23}
g) 4(2) cage at R6C3 = {04/13}
h) 15(2) cage at R7C3 = {69/78}
i) 8(2) cage at R7C4 = {08/17/26/35}
j) 12(2) cage at R7C7 = {39/48/57}
k) 7(2) cage at R8C5 = {07/16/25/34}
l) 8(2) cage at R9C3 = {08/17/26/35}
m) 11(2) cage at R9C6 = {29/38/47/56}
n) 6(2) cage at R10C4 = {06/15/24}
o) 8(2) cage at R10C5 = {08/17/26/35}
p) 6(3) cage at R5C5 = {015/024/123}
q) 23(3) cage at R5C9 = {689}
r) 3(3) cage at R7C6 = {012}
s) 8(3) cage at R7C11 = {017/026/035/125/134}
1a. Naked triple {689} in 23(3) cage at R5C9, locked for N4, clean-up: no 2,3,5 in R4C8
1b. Naked triple {012} in 3(3) cage at R7C6, locked for N6, clean-up: no 5,6,7 in 7(2) cage at R8C5, no 6,7,8 in R9C3, no 9 in 11(2) cage at R9C6
1c. Naked pair {34} in 7(2) cage at R8C5, locked for C5 and N6, clean-up: no 5,6 in R3C6, no 5,6 in R5C4, no 5 in R9C3, no 7,8 in 11(2) cage at R9C6, no 2 in R10C4, no 5 in R11C6
1d. Naked pair {56} in 11(2) cage at R9C6, locked for R9, clean-up: no 2,3 in R9C3
1e. 4(2) cage at R4C7 and 6(3) cage at R5C5 form combined 10(5) cage {01234), locked for N3, clean-up: no 7 in R4C9
1f. 11(2) cage at R4C8 = [74/83/92] (cannot be [65] which clashes with R4C56)
1g. This step held until step 2f.
[Time to look at semi-symmetry.]
2a. 14(2) cage at R4C5 = {59/68} corresponds with 11(2) cage at R9C6 = {56} -> at least one of 8,9 must be paired with one of 5,6 -> 5,6 cannot be paired with each other
2b. R49C6 are in the same column and not in repeat cells -> R4C6 = {89}, R4C5 = {56}
2c. Whichever are in R49C6 must be paired, R4C5 and R9C7 must be the same since 5 not paired with 6 -> 5 must be paired with 8 and/or 6 must be paired with 9
2c. 4(2) cage at R4C7 = {04/13} corresponds with 7(2) cage at R8C5 = {34} -> 0 must be paired with 3 and/or 1 must be paired with 4
2d. R4C9 corresponds with R9C3 = {01}, no 0,1 in R4C9 -> R4C9 = {34}, clean-up: no 9 in R4C8
2e. 23(3) cage at R5C9 = {689} corresponds with 15(3) cage at R7C1
2f. 15(3) cage at R7C1 overlaps with R78C3 = {69/78} -> 15(3) cage cannot be {069/078}, no 0,9 in R7C1 + R8C2
2g. 15(3) cage cannot contain both of 6,9 -> 6 cannot be paired with 9 -> 5 must be paired with 8
2h. R49C6 = [85] -> R4C5 = 6, R9C7 = 6, clean-up: no 9 in R2C5, no 1 in R3C5, no 3 in R3C6, no 3 in R5C4, no 0 in R10C4, no 0 in R10C5, no 2 in R11C6
2i. R4C8 = 7 -> R4C9 = 4, clean-up: no 0 in R5C7, no 1 in 5(2) cage at R5C8
2j. R4C8 = 7 corresponds with R9C4 -> R9C4 = 7 (because 8 paired with 5) -> R9C3 = 1, clean-up: no 3 in 4(2) cage at R6C3, no 2 in R6C5, no 5 in R7C7
2k. R4C9 corresponds with R9C3 -> 1 and 4 are paired; don’t yet know whether 0 and 3 are also paired
2l. Naked pair {04) in 4(2) cage at R6C3, CPE no 0,4 in R6C2
2m. 23(3) cage = {689}, 5 paired with 8 -> 15(3) cage must contain one, but not both, of 5,8 = {348/357/456}, no 2,9, clean-up: no 6 in R7C3
[Note that {258} would also be eliminated by clashing with R78C4.]
2n. 15(3) cage = {357} (cannot be {348/456} because 1 paired with 4) -> R8C3 = 7, R7C1 + R8C3 = {35}, locked for N5, R7C3 = 8, clean-up: no 0 in 8(2) cage at R7C4, no 4 in R8C8
2o. 5 paired with 8, 5 in R7C1 + R8C3 corresponds with 8 in R5C10 + R6C11, 8 locked for 23(3) cage at R6C10
2p. R8C3 = 7 corresponds to R5C9 = {69} -> 7 paired with one of 6,9, 3 paired with the other of 6,9 -> 0 and 2 must be paired
2r. Naked pair {26} in 8(2) cage at R7C4, locked for C4 and N5, clean-up: no 7 in R6C5, no 0 in R11C5
2s. R4C4 corresponds with R9C8 -> R4C4 = 9, R9C8 = 9 (they cannot be R4C4 = 5, R9C8 = 8 because then 9 would be the missing number from both N3 and N6), clean-up: no 0 in R6C5, no 3 in R7C7
2t. 5 not in N3, 8 not in N6; both of 5,8 must be in the other nonets
2u. R2C5 corresponds with R11C7, R2C5 = {58} -> R11C7 = {58}
2v. R7C3 corresponds with R6C9, R7C3 = 8 -> R6C9 = 5, clean-up: no 4 in R5C4, no 0 in 5(2) cage at R5C8
2w. Naked pair {23} in 5(2) cage at R5C8, locked for C8 and N4, clean-up: no 1 in R2C7, no 9 in R7C7
2x. 1 in R7, apart from possible repeat in R7C6, only in R7C8910,11, locked for N7
2y. 8(3) cage at R7C11 = {026/035/125} (cannot be {017} because 1,7 only in R7C11, cannot be {134} which clashes with R8C5), no 4,7
3a. R8 has 9 cells without 9 -> R8C8 = 8 (hidden single in R8) -> R7C7 = 4, R7C2 = 0 -> R6C3 = 4, R7C5 = 9, clean-up: no 0 in R3C6, no 0 in R4C7, no 0 in R5C4
[The last key step, fairly straightforward from here.]
3b. Naked pair {13} in 4(2) cage at R4C7, locked for C7 and N3, R2C7 = 0 -> R3C8 = 4, R7C7 = 7, clean-up: no 5 in R3C5, no 9 in R3C6
3c. R6C7 corresponds with R7C5 -> 7 paired with 9 -> 3 with 6
3d. No 4 in N5 -> no 1 in N4 -> R6C10 = 0
3e. 1 in R6 only in R6C1245, locked for N2 -> 9(2) cage at R5C4 = [81] -> R6C4 = 3, 5(2) cage at R5C8 = [32], 4(2) cage at R4C7 = [31], clean-up: no 5 in R10C4, no 7 in R11C6
3f. R8C3 = 7 corresponds with R5C9 -> R5C9 = 9, R5C10 = 6, R6C11 = 8
3g. R6C11 = 8 corresponds with R7C1 -> R7C1 = 5, R8C2 = 3, 7(2) cage at R8C5 = [43]
3h. R8C7 = 2 -> 8(2) cage at R7C4 = [26], R8C910 = [05] -> R7C11 = 3 (cage sum), R78C6 = [01], clean-up: no 8 in R3C5, no 7 in R10C5
3i. No 9 in N7 -> no 7 in N2
3j. R7C11 = 3 corresponds with R6C1 -> R6C1 = 6, R5C23 = 7 = {25}, locked for N2, 2 locked for R5 -> R5C56 = [04], R6C6 = 2
3k. R3C5 = 7 (hidden single in C5) -> R3C6 = 2
3l. R10C4 = 4 (hidden single in N8) -> R11C5 = 2, clean-up: no 6 in R11C6
3m. R2C6 corresponds with R11C6 -> R2C6 = 6 -> R2C5 = 8, 8(2) cage at R10C5 = [53]
3n. R10C8 = 1 (hidden single in N8)
3o. R1C6 = 9 corresponds with R12C6 -> R12C6 = 7
3p. R6C2 = 9 corresponds with R7C11 -> R7C11 = 7
3q. R8C9 = 0 corresponds with R5C3 -> R5C3 = 2
and the rest is naked singles.