There seems to be a general consensus that classical Egyptian had four "guttural" or "H-like" phonemes: h (building,
/h/), ḥ (wick,
/ħ/), ḫ (placenta?,
/x/), and ẖ (animal's belly,
/ç/). And the main works that I've seen exploring the historical phonology (e.g. Loprieno) haven't proposed any additional ones, to my knowledge.
However, the documentation for CLTK's MdC transliterator seems to mention a fifth:
Small and Capital H5: almost exclusively used for transliterating demotic script.
What is "H5"? Is this a different name for one of the "standard" phonemes, or a new discovery, or a fringe theory that isn't accepted by the mainstream? Or something else entirely?