Okay.. so for anyone looking for a genuinely legal interpretation, I found no reference above that actually quote LEGAL DICTIONARY sources.. so here goes..
Black's Law Dictionary, 5th Edition has the following instances of the options requested above:
seven (7) references for the word "signer"
only two (2) references for the word "signatory"
(and in case anyone's wondering.. there is no word spelled "signator". This is verifiable using both Microsoft Word as well as here posting a reply using WordReference.com's software.)
Furthermore, the following are found in the specific word definition areas.
-- Signer: (Strangely, even though the word is used 7 times, they actually do not have a definition reference for the word "signer".
-- Signatory /sign;t(o)riy/.
A term used in diplomacy to indicate a nation which is a party to a treaty.
In general, a person who signs a document personally or
through his agent and who becomes a party thereto.
BUT TO REALLY HELP YOU OUT ANY NEWCOMERS TO THIS THREAD, I took some time to literally find and read each of the specific mentions (located within Black's Dictionary) of the two words above, and what I found should easily enable anyone to make a proper distinction.
- EVERY use of the word "signer" was applied to referencing the specific person who PHYSICALLY MAKES (CREATES) a signature on a document, whereas...
- the one reference to the word 'signatory' refers only to the POWER (or AUTHORITY) someone POSSESSES for the purpose of ruling whether or not a signature is lawfully valid with regard to any signing of a specific (named) document type.
Black's Law Dictionary is the formal reference for Judges from all English speaking nations. And that enables me to believe that if this is sufficient guidance for Supreme and lower court judges, the rest of us should likely be equipped quite well now, to draw a successful conclusion from what I have posted here.
My take is that the question above, itself, is actually written in a manner that is ambiguous enough that it offers both of these words to be the correct answer.
Although, it did seem to me, that the original person posted this question was leaning more toward asking what the actual person DOING the signing should be called,
I the submit the following summary:
Because the word 'signatory' only refers to the official legal POWER a person needs to be appointed with in order to BE the one who affixes his/her signature to a certain specific type of document,
then using the word 'signatory' as a reference to someone who was (or is going to to be) the person who physically executes the act of signing, would be an INCORRECT use of the word; whereas, contrarily, the word 'signer' is definitely the appropriate word to employ.