Yours aye

I am curious to know what does the phrase 'Yours aye' at the end of a letter really mean. I know 'aye' means yes but to say 'Yours aye' in place of 'Yours sincerely' or 'Yours faithfully' does not quite make sense. Please note English is not my mother tongue

One meaning of "aye" is forever, sometimes used in "for aye and aye." I hever heard it (or read it) as the "complimentary close" of a letter. I would have expected it to be rendered "Yours for aye," if at all.
SS