G09B 3/045
With invisibly printed correct answer
Official title: {with invisibly printed correct answer; the answer being rendered visible by special optical means, e.g. filters; mirrors}
G09B3/045 (With invisibly printed correct answer) appears on 2 US grants and 2 published applications in this corpus.
Search patents in G09B- Granted patents
- 2
- Published applications
- 2
- Assignees
- 2
- Inventors
- 15
- Latest grant
- 18 Feb 2020
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