I’ve used W1, W2, W3 and W4 for the windows at R2C2, R2C6, R6C2 and R6C6. The hidden windows are given in full; they are explained in my post in the Standard Techniques forum
here. I didn’t start using the hidden windows until I needed to.
Prelims
a) R12C2 = {15/24}
b) R23C4 = {39/48/57}, no 1,2,6
c) R23C6 = {59/68}
d) R4C23 = {15/24}
e) R4C78 = {69/78}
f) R6C23 = {14/23}
g) R6C78 = {59/68}
h) R78C4 = {15/24}
i) R78C6 = {17/26/35}, no 4,8,9
1. R4C23 = {15} (only remaining combination, cannot be {24} which clashes with R6C23), locked for R4, N4 and W1, clean-up: no 1,5 in R1C2, no 7 in R23C4, no 4 in R6C23
2. Naked pair {24} in R12C2, locked for C2 -> R6C23 = [32], placed for W3, no 4 in R78C4
3. Naked pair {15} in R78C4, locked for C4, N8 and W3, clean-up: no 3,7 in R78C6
4. Naked pair {26} in R78C6, locked for C6, N8 and W4, clean-up: no 8 in R23C6, no 8 in R6C78
5. Naked pair {59} in R23C6, locked for C6, N2 and W2, clean-up: no 3 in R23C4, no 6 in R4C78
6. Naked pair {48} in R23C4, locked for C4, N2 and W1 -> R2C2 = 2, placed for D\, R1C2 = 4
7. Naked pair {78} in R4C78, locked for R4, N6 and W2
8. Naked pair {59} in R6C78, locked for R6, N6 and W4
9. 7 in W1 only in R2C3 + R3C23, locked for N1
[With hindsight, step 23 could have been done at this stage but I didn’t spot it until later. Similarly for the other three quadrants. This would have slightly shortened my solving path.]
10. 1,2,6 in W2 only in R23C78, locked for N2
11. 4,8,9 in W3 only in R78C23, locked for N7, 4 also locked for C4
12. 3 in W4 only in R78C78, locked for N9
13. 2 in Old Lace only in R3C5 + R5C7, CPE no 2 in R3C7
13a. R3C8 = 2 (hidden single in N3)
13b. R5C7 = 2 (hidden single in Old Lace)
13c. R4C5 = 2 (hidden single in N5)
13d. R1C4 = 2 (hidden single in R1)
13e. R9C1 = 2 (hidden single in R9)
13f. 6 in N2 only in R123C5, locked for C5
14. R5C5 = 5 (hidden single in Old Lace/R5/C5), placed for both diagonals
14a. 9 in N5 only in R45C4, locked for C4, CPE no 9 in R5C3 using Old Lace
14b. 6 in N5 only in R456C4, CPE no 6 in R5C3 using Old Lace
15. R59C2 = {15} (hidden pair in C2)
16. 1 in Old Lace only in R3C5 + R6C6, CPE no 1 in R6C5
16a. 1 in C5 only in R123C5, locked for N2
17. 4 in Old Lace only in R46C6 + R7C5, CPE no 4 in R6C5
17a. 4 in C5 only in R789C5, locked for N8
[With hindsight the eliminations in steps 16 and 17 could have been made after step 14b using the Old Lace property R37C5 = R5C46 and R5C37 = R46C5.]
18. 1 on D/ only in R2C8 + R3C7, locked for N3
19. 3 in W1 only in R23C3 + R4C4, CPE no 3 in R1C1 using D\
20. 1 in R6 only in R6C69, CPE no 1 in R9C9 using D\
21. 9 in R4 only in R4C14, CPE no 9 in R1C1 using D\
22. Naked quad {1346} for W2 in R2C78 + R3C7 + R4C6, CPE no 3 in R1C9 using D/
22a. 3 on D/ only in R2C8 + R3C7 + R4C6, locked for W2, no 3 in R2C7
23. Naked quad {3679} for W1 in R2C3 + R3C23 + R4C4, CPE no 6 in R1C1 using D\
24. Naked quint {13478} for W4 in R6C6 + R78C78, CPE no 4,7,8 in R9C9 using D\
25. Naked quad {3469} in R4C1469, CPE no 3,6 in R3C5 using Old Lace and hidden window R234C159
26. Naked quint {14678} in R6C14569, CPE no 4,7,8 in R7C5 using Old Lace and hidden window R678C159
27. Naked quad {3679} in R4569C4, CPE no 7 in R5C3 using Old Lace and hidden window R159C234 -> R5C3 = 8
27a. R6C5 = 8 (hidden single in N5), placed for hidden window R678C159, no 8 in R78C9
27b. R9C6 = 8 (hidden single in C6), placed for hidden window R159C678, no 8 in R1C78
28. 4 in Old Lace only in R46C6, locked for C6
28a. 6 in Old Lace only in R46C4, locked for C4
29. 4 on D\ only in R6C6 + R7C7 + R8C8, locked for W4, no 4 in R7C8 + R8C7
30. Using Old Lace R3C5 “sees” all 1,7 in C6 except R5C6 -> R5C6 = R3C5 = {17}
31. Using Old Lace R7C5 “sees” all 3,9 in C4 except R5C4 -> R5C4 = R7C5 = {39}
32. Using Old Lace R3C5 = {17} “sees” all 1,7 in R6 except for R6C9 = 1 or R6C1 = 7 -> no 6 in R6C9 (because R6C4 = {67})
33. Using Old Lace R7C5 = {39} “sees” all 3,9 in R4 except for R4C9 = 3 or R4C1 = 9 -> no 4 in R4C1 (because R4C6 = {34})
[Continued further use of the hidden windows, but the puzzle is now cracked.]
34. 4 in hidden window R234C159 only in R234C9, locked for C9 -> R6C9 = 1, placed for hidden window R678C159, no 1 in R78C1
34a. R5C6 = 1 (hidden single in R5), placed for hidden window R159C678, no 1 in R9C78
34b. R3C5 = 1 (hidden single in Old Lace), placed for hidden window R234C159, no 1 in R2C1
34c. R2C8 = 1 (hidden single in N3)
34d. 7 in R5 only in R5C12, locked for N4
35. R1C1 =1 (hidden single in C1), placed for D\
35a. R1C9 = 8 (hidden single in R1), placed for D/
35b. R7C2 = 8 (hidden single in C2)
35c. R8C8 = 8 (hidden single on D\), R4C78 = [87]
36. R7C8 = 3, R7C5 = 9, placed for hidden window R678C159, no 9 in R8C9
36a. R5C4 = 9 (hidden single in N5), placed for hidden window R159C234, no 9 in R1C3
36b. R4C1 = 9 (hidden single in N4), placed for hidden window R234C159, no 9 in R23C9
36c. R5C9 = 3 (hidden single in R5), placed for hidden window R159C159, no 3 in R19C5
36d. R9C9 = 9 (hidden single in C9), placed for D\
36e. R8C2 = 9 (hidden single on D/)
36f. R8C7 = 1 (hidden single in N9), R78C4 = [15]
36g. 7 on D/ only in R6C4 + R7C3, locked for W4, no 7 in R8C3
37. R3C3 + R4C4 = {36} (hidden pair on D\), locked for W1 -> R3C2 = 7, R2C3 = 9, R23C6 = [59], R5C2 = 6, placed for hidden window R159C234, no 6 in R19C3, R56C1 = [74], R6C6 = 7, R7C7 = 4, R6C4 = 6, R7C3 = 7, R4C4 = 3, R3C3 = 6
and the rest is naked singles, without using diagonals or windows.
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