Hi Rosanne!
My guessing is that PCS was asked by Waite or the editor of Rider Co. not to put her monogram in the Fool drawing. A possible reason for the request might be that they were planning to use the image for many ways including box, frontboard, poster etc. PCS agreed, but secretly put her monogram among the lines of the cliff (she liked such trick. "PAX" of 4Swords is an example). But the copyst in the lithographic printing process failed to recognize it and did a rough rendering ; now what we see is a remains of PCS monogram.
I know it's just a gussing without hard evidence. I can offer another wilder version if you like
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Now I cancell my posts above on the " horizontal signature hidden in the cliff of the Fool". My present pet theory is "originally the drawing had been a little longer vertically, and the bottom space where PCS monogram stayed was later covered by a piece of paper". My guessing is that first PCS drew several trumps in full size 100mmx177mm, then she was told the size includes the title space. Re-drawing was unpractical so the emergency-aid was applied; a patch of paper was put over the bottom to make a title space. I believe several trumps ie The Fool, The Magician, The Sun and The Moon were modified thus. Other cards were okay from the start.