Luther informed Philipp Melanchthon on 15 September 1530, that the Prince had personally visited him in the Coburg fortress and presented him with a signet ring, presumably displaying the seal.
Luther's doctoral ring displayed a heart-like shield, the symbol of the Holy Trinity.
[3] In an 8 July 1530 letter to Lazarus Spengler, Luther interprets his seal: Grace and peace from the Lord.
The first should be a black cross in a heart, which retains its natural color, so that I myself would be reminded that faith in the Crucified saves us.
Such a heart should stand in the middle of a white rose, to show that faith gives joy, comfort, and peace.