This is using llythyr as a note/memo/posted message. Might sound stupid, but can "Llythyr" be used as in letter as a part of a word? As in "You spelled that wrong, you used the wrong letter"?
llythyr (llythyrau) (masculine) - letter (letters) that you post.
llythyren (llythrennau) (feminine) - letter (letters) of the alphabet (note that the second y is dropped in the plural).