Henry IV of England used a "remarkable seal" with two tall feathers (around the shield, helm and crest), each entwined with a garter which had his "favorite and significant" motto "SOVEREYGNE" written on it, which he is said to have used "a short time before his accession", as Duke of Lancaster. (Sources: English Heraldry, by Ch. Boutell, pg. 242. A Complete Guide to Heraldry, by Arthur Charles Fox-Davies)
A modern depiction of this feather can be seen here, by "Sodacan" on wikicommons.
Can anyone find a photo or even illustration of this image? I can't for the life of me find one. I can find double ostrich feathers for the Duchy of Lancaster, just not entwined with "SOVEREYGNE".
I came across this Seal of John, Duke of Bedford, which looks like it has the ostrich feathers entwined with writing. John of Lancaster was the third son of Henry IV, but I can't find any other higher resolution photos of this to confirm if this is the same symbol