The New Sudoku Players' Forum

[dobrichev]

https://www.google.com/patents/US20090014953 from July 2007

Enjoy!

[JasonLion]

The patent describes a device to keep track of the pencil marks in a single cell, allowing you to check for naked singles, designed so it can be reused for each of the unsolved cells sequentially.

It sounds extremely tedious to use, as you need to rediscover the on/off state of each pencil mark for each cell manually each time through.

[dobrichev]

This can be bundled with a kit of 729 pencilmarks. Plain for the Sport edition and coloured for the Heavy Duty edition.