hubby11
The accompanying sabot is still a mystery
That is a Knight Sabot it is a thick devil for the older Knights. Knight sold there sabot machines to Del (MMP) but he still makes that dabot for them to their specification. It will probably not load down any of the modern guns with a tight bore... If you look at the petals you can see that their is a wider gap than a normal MMP sabot between the petals. Think it measures .508 - I use them in my White.
It is an regular Hornady XTP... the picture at the beginning of your thread I beleive has been correcly identified as a 260 Nolser. I knew it wasn't a Hornady..