A small guess: If it was written over a line of paper's imposition, in such a way that if you unfold the envelop the upper part of the signature goes far away from the bottom part. Such a signature sometimes was used as an improvised check that the envelope had not been unfolded during delivery, because if it was the signature would had been cracked along the imposition line. Though there is no any guess why he used these specific letters for this purpose.