'Yours faithfully' as specifically used for cases in which the name of the addressee is not known, is a British formality. I do not know how strictly it is now adhered to there, but in AmE it sounds very antiquated, and for most letters, common standards are 'Sincerely yours' and 'Yours truly', with their permutations. Any of these would be entirely satisfactory for a TWIMC letter.
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