The identity of the photographer listed only as X ★★★ on the tissue-guard and Index (Table) page for this publication has not been determined at this time.  Photographic pseudonyms were sometimes purposefully used in publications during this time as a way to promote discussion, criticism, and of course, speculation and interest as to who the real author was.
Our own speculation might lead to Belgian photographer Romain Ickx, but only if the publishers had his consent to transcribe the first three letters of his last name and appended them to the end instead.