whitetailcrazy
They are MMP, made under contract for Barnes and they are EZ glides in some rifles, like older Knights and .504 Whites. The sabot is about the same size as a HPH-12. The other thing that you brought out - they are made for a boattail bullet, also look at how short the the slit between petals is... this helps hold the boattail in place.
MMP does not offer a sabot to the public that will replace the EZ glide so you can shoot a thinner sabot and a barns boattail bullet. Anyway the last time I asked they were not making a public available boattail sabot.