Comments from Submitter:
Unfortunately I have no source, but a very special memory for the set 6085. I confirm that in instructions (pictures and photos) that knight has no cape, but I am convinced that there were in the set that I bought at the time (2 of the 4 knights had a cape). Lately I bought this set again from ebay (95% complete) and then the minifigure also wore a blue cape.
I imagine it may be an inventory variation. I bought this set in France in the last year of its production (it was no longer in the following catalog). The one I bought on ebay also comes from France.
I don't know how to confirm this. I noticed that in the inventory history, the minifigure had been recorded with a blue cape, and that it was remove in 2008.
[...] This request will not be accepted. Without better proof than recollection and a used/incomplete set, we must rely on what is available (the instructions and set photos). [...] (41 months ago, Feb 3, 2021, to Inventories Requests)