What is "privy signet"

Signet \Sig"net\, n. [OF. signet a signet, F., a bookmark, dim. of signe. See Sign, n., and cf. Sennet.] A seal; especially, in England, the seal used by the sovereign in sealing private letters and grants that pass by bill under the sign manual; -- called also privy signet.

I had my father's signet in my purse.
--Shak.

Signet ring, a ring containing a signet or private seal.

Writer to the signet (Scots Law), a judicial officer who prepares warrants, writs, etc.; originally, a clerk in the office of the secretary of state.