Webster’s 1913 has no entry under this spelling.
Shades of meaning
Fernald has no entry of its own for unlettered; he lists it under ignorant.
Where he writes about it:
Unlettered is similar in meaning to illiterate, but less absolute; the unlettered man may have acquired the art of reading and writing and some elementary knowledge; the uneducated man has never taken any systematic course of mental training. — from ignorant