WordNet records no synonym and no antonym for letters testamentary. It is the only member of its synset, which is common for technical, taxonomic and single-sense vocabulary — there is no other English word that means the same thing. The broader and narrower terms below are the useful relations in that case.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
a legal document from a probate court or court officer informing you of your appointment as executor of a will and empowering you to discharge those responsibilities
Synonyms, antonyms and the broader and narrower relations for letters testamentary come from Princeton University’s WordNet. Full sources and licences.