Were they in labeled packaging when purchased, or were they sold with bullets?
Maybe it's a custom or proprietary sabot that was molded by one of the major sabot manufacturers for a single company. For instance MMP makes custom .52 sabots for Knight rifles which only Knight sells.
It's possible that if the folks at MMP or Harvester were asked that they might recognize that sabot as either one of their own or a competitor's.
Someone at each of those companies should be available to speak with you on a personal basis, and then the photo can be emailed to them.
Here's two major manufacturers and perhaps if you speak with them they can tell you about others to contact if they can't help you: