3
x x 9  x 3 2  x x x
x x x  7 x x  x x x
1 6 2  x x x  x x x

x 1 x  x 2 x  5 6 x
x x x  9 x x  x x x
x 5 x  x x x  1 x 7

x x x  x x x  4 x 3
x 2 6  x x 9  x x x
x x 5  8 7 x  x x x

<https://sudoku.com/sudoku-rules/hidden-singles/>
The backtracking number is grid331 in just naked singles. Adding hidden singles it's grid72. There are hidden singles right off in col3 (1), col4 (2), box7 (1), box8 (2). One of the start hidden singles is both in box7 and col3, i3. `sudoku.com` uses box7 but my solver uses col3 because it goes arbitrary row, col, box. I don't think it matters because it immediately starts looking for naked singles in the mask again.
4 7 9 ║ 1 3 2 ║ 6 8 5 
5 3 8 ║ 7 6 4 ║ 2 1 9 
1 6 2 ║ 5 9 8 ║ 7 3 4 
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9 1 3 ║ 4 2 7 ║ 5 6 8 
6 8 7 ║ 9 1 5 ║ 3 4 2 
2 5 4 ║ 6 8 3 ║ 1 9 7 
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8 9 1 ║ 2 5 6 ║ 4 7 3 
7 2 6 ║ 3 4 9 ║ 8 5 1 
3 4 5 ║ 8 7 1 ║ 9 2 6