Researching a Mystery - thank goodness for google

Among all the usual everyday items of ages past occasionally something turns up that is different and mysterious. This is a tale of such an item, uncovered in my local charity shop, hidden in a stack of picture frames, it screamed out in its oddness.
A certificate, a document, a piece of (someones) history.
In a thin wooden frame, mounted on faded red card, shrivelled with age, it reminded me of a museum exhibit

for a pound.

On closer inspection I could see it was vellum, the traditional writing surface of ages old, stretched and treated calf skin, with words in French, old French not modern, written in beautiful calligraphy, embellished with swirls, decorated with a faded ribbon, displaying a large red wax seal showing the resplendent figure of a mounted knight.

This is interesting.

After googling a French interpretation site to try and translate the words, two didn't come up, now obsolete from the language. But I got the gist of the text.

Titled - Order of the Grand Dukes of the West.

A bit of research later and I found them, started by the first Grand Duke (ranked below the King) Philip 111, Grand Duke of Burgundy in 1435.
I'm getting excited now, so take it to the local auction house for expert advice, the auctioneer said in all his years he'd never seen such a thing but couldn't tell me anymore about it, other than it was possibly dated around the 1920's as on closer inspection the calligraphy was printed into the vellum, only the recipients name was inked in, slightly faded with age.
Curiouser and curiouser...what was an ancient style French certificate, printed on expensive vellum doing in a Sussex charity shop?
I asked one of my students George Gush, retired Head of History at West Kent College, but although he was an expert and had written books on the subject, he only knew historical facts and couldn't provide me with any more information on this specific document.

More research, after scouring the Internet I contacted the British Library, they couldn't help me as it was French, then the Louvre, getting a very curt reply saying they don't give out information to the general public...now I'm beginning to think this may be a secret sect!
I contacted a group of medieval minstrel players in Burgundy who mention the Order of the Grand Dukes on their web site. They sweetly replied that indeed there still was an annual gathering, each year in a different French city, kept very secret with only the most important and powerful members invited. Suggesting perhaps this certificate was presented to a new member or orator at one such event.

My research stopped there, quelled by the secrecy of an ancient club similar or affiliated to The Order of the Golden Garter or Fleece.

So if anyone knows any more information about this strange document I'd love to hear it.