As I have demonstrated and mentioned in some previous posts, this pattern is most familiar with late (1920s) pumtek beads, but is also seen in earlier decorated agate beads. It is logical the latter inspired the former. According to suggestions, modern pumtek bead were made to copy (or were inspired by) Pyu beads. This may be a simplification. In other words, the folks of this region may have been familiar with a larger corpus of decorated beads than just Pyu beads, and the interest in these beads may have continued through the years (not necessarily revived from nothing centuries later).

But it would be difficult to say much that would be more specific.

Jamey