Wotan, Great thread on this rare Diplomatic portepee. There are many mysteries still to learn in this hobby, this unusual and obviously rare portepee being a prime example.
When something like this turns up it's natural to most to say it's a fake or unknown, but certainly not a Diplomatic portepee. Why?, because they never seen one in person, or in a book, or on the internet.
Experienced long time collectors do further in-depth research, as you have. They ask the long time volume dealers if they ever seen one. Well, it appears that they have, as has been indicated by Tom Johnson, Tom Wittmann and recently Ron Weinand.

Next we look at the quality of the manufacture for further clues if perhaps it the item conforms to materials and dimensions of what we know and accept as 'period made' examples.
Your comparison photo next to a Army and G.O./Diplomatic portepee shows that it indeed is the proper size for the smaller portepee used on G.O./Diplomatic daggers.
Those that have researched know that there is yet to be found a period photo of one in use. Does that mean they didn't exist ? No, all it means is that we have yet to see one, perhaps one day we will, as in the case of the early so called HJ Honor knifes. We also know that there are period in-wear photos of obscure daggers that have yet to surface post-war. However, IMO one has to be open to a example surfacing one day as on rare occasions they still do. Recently at the last SOS a rare Army prototype dagger by Eickhorn walked through the door. It was snagged by a dealer and later was inspected my myself and several other experienced collectors who all knew it was 100% correct and very rare. Not one like that has ever been shown in a publication.

Recently on WAF a member collector bought a Diplomatic dagger from a grandson of the original owner that had this exact portapee attached to to the dagger. Thinking it was not proper (perhaps fake) he left the portepee with the owner who later sold it at auction. I don't know if links are allowed here, is so, I can link the thread here. A interesting story.

When at times I'm asked about something 'unusual' concerning daggers these days my reply usually is: No, I never seen one like that, but all that means is; 'I never seen one'.
Reminding the questioner that IMO, half of the artifacts have been either lost or destroyed during or after the war, so there are many items we will never see, doesn't mean they never existed.

Regards,
-Serge