It looks man-made to me. There were a number of razor hones that emulated the bbw/coticule combination hones, such as Nu-Way and Frictionite. The meandering line between the two halves seems to have been caused by the dark half being allowed to set in the mould, then having the yellow half poured on top - any contraction of the bottom layer away from the sides of the mould would generate this type of seepage. Should be easy to check - scratching or drilling into the lowest part of the yellow layer that overlaps the dark layer should reveal the dark layer pretty quickly.

You can see the uneven line in the following pics of frictionite hones - low quality because they were grabbed from the web:

Name: frictionite.jpg Views: 274 Size: 45.2 KBName: frictionite2.jpg Views: 195 Size: 26.4 KB

Regards,
Neil