Lewis & Short

obsignātor, ōris, m. [obsigno], a sealer, sealer up (class.): litterarum obsignator, Cic. Clu. 66, 186.
Esp., the witnesses who set their seals to a will, Cic. Clu. 13, 37: testamenti, id. Att. 12 18, b, 2: decisionis, id. Fl. 36, 89.