Jez,
Thanks for the kind words. Say "Photos by Mac. These photos first appeared on The Prop Den" and perhaps a link to the Den.
I spoke with Rubie's at one point regarding their Fiberglass Limited Edition Darth Vader. The gentleman (the owner?) said they "leased" the "same moulds" used by Don Post Studios.
I have also observed that the Don Post Standard Vader has become the Hasbro Voice Changer as well as the Rubie's kids' half-mask. It's also become the Rubie's kids' Vader costume half-mask.
Seeing the same sculpt appear across different brands leads me to think that when Don Post's license expired, Lucasfilm took possession of their moulds and thus repurposes the moulds depending on who wants to become a licensee. It all makes sense. If you create different categories of accuracy (moulds) then you can have different categories of licensing. That makes MR's former line and eFX's emerging line more the cream-of-the-crop while the Rubie's are to us accuracy fanatics the crud-of-the-crap (depite the inaccuracy, I still love Rubie's as a company. I'm glad someone picked up the license!)
I think the license cost Rubie's enough to where if they didn't have to commission a sculptor, they could just make use of a costume sculpt that LFL already had in their possession. The goal of a costume company like Rubie's isn't to achieve accuracy. It's to sell to the mass market. Their Deluxe/Supreme Vader mask shows quite a difference to the Don Post Classic Action Vader, but it strikes me that its particular inaccurate shape makes it easier to pull from its molds. Rubie's has to sell perhaps 50 of these to make the profit of one eFX or MR helmet.
Jez, I have a feeling this 4-piece Model 2868 is not the Supreme.
Here's a photo I found on the web of, allegedly, the Supreme:

So I did some further analysis.

The Model 2868 is similar but not identical to the Supreme. The 2868's head is two halves. The Supreme's head (not counting the ears) is a one-piece construction and a slightly different paint scheme to the mouth. The eyes are darker. Lastly the blue stripes on the 2868 is a sticker that is placed into a rectangular depression. I'm not certain that the Supreme has that feature.