You may have noticed that some colors copy better than others on Xerox and other copying machines that you’ve used. While this phenomenon is sometimes an annoyance, it has practical applications, such as not reproducing alignment lines and written comments.
There are, of course, specialized pencils for these applications. There have been many different copying technologies and purposes, and pencils designed for optimal performance in each situation were produced.
Dixon produced an extensive “Fax” line of such pencils. I have a set that came in a sales folder with information sheets —
Here are some double-ended varieties from other manufacturers:
(The No. 625 is from Richard Best.)