Overprint

Philatelists are familiar with overprints on old postage stamps.  Often they were used to alter the postal value.  A similar concept was applied to a pencil —

You can see that the original No. 2 imprint was changed to 3 —  

Apparently the manufacturer realized that the 2 was an error and made this correction.  Another possibility: They made these to fill a rush order for No. 3s, hoping the customer wouldn’t notice anything suspicious.