"If any of your correspondence could favour the public with an interpretation it would oblige many of your readers here"
Henry Percy's seal did indeed depict a lion, three of them in fact, over a shield and plumed helmet. The Percy Arms, as shown on the seal of Henry de Percy 1st Baron Percy who died in 1314, does show one lion rampant. With all this evidence it is understandable why its associated with the Percy's.
However, it probably did not belong to Henry Percy as we can dismiss the lion, many medieval families used it on their insignia, after all its a Plantagenet icon, Henry II and his son John probably used it and we know it was on the Great Seal of Richard I. Also, there is no evidence that the motto "Now is Thus" was ever used by the Percy family.
Whoever owned this ring was a high-status individual who, not only lost his ring but probably his life, along with twenty-eight thousand other men, on that day at Towton in 1461.