We construct this by adding eight sets of 1-9: r3, r4, r6, r7 c3, c4, c6, c7 then taking away a different eight sets of 1-9 r1, r2, r8, r9 c1, c2, c8, c9
This tells us that the digits that appear in the light blue area, plus two instances of each digits that appear in the dark blue area are the same digits making up the set formed by the digits in the light green area plus two instances of each digits that appear in the dark green area
Even if the light greens contain no digits of 5-8, we know that the light greens contain some, and that the light blues don't THEREFORE the dark blues must contain at minimum a 5, a 6, a 7 and an 8
We've already been given the corner digits (1, 2, 3, 5) So the question then becomes: Can we fit MORE than three digits larger than 4 into the remaining cages (8, 12, 14, and 11) ?
4. Cage sums
8 in 3 (without a 5) = 134
11 in 3 (without a 1, and with at least one of 6, 7 or 8) = 236 12 in 3 (without a 2, and with at least one of 7 or 8) = 138 147 (so must contain a 1) 14 in 3 (without a 3, and with at least one of 7 or 8) = 158 167 248 257 (so must contain either 1 or 2)
5. Narrow down the possibilities
In box 1, the 3 in the 8 cage is forced into row 2, eliminating any possible 3 from the 12 cage in box 3 So that's now 147
In box 1, the 1 in the 8 cage and the 1 in the 3 cage in box 4, eliminates any possible 1 from the 14 cage in box 7 So that's now 248
Now we know the numbers in the light blue, dark blue and light green (if not which one is where), we know exactly which digits must be in dark green, and which are not. All the 5678 required are accounted for, so the remainder (the dark greens) MUST all be 1-4
Furthermore, we can say how many there are of each: 1234 (from light blue, halved) + 134 + 124 + 234 + 123 means there are exactly four "1"s, four "2"s, four "3"S and four "4"s in the dark green areas.
All this, without using the clock lines at all, so far.
7. Fill in the remaining pencil marks
The white spaces are 56789, though we can eliminate 5 from r1 and c1