U+0373 Greek Small Letter Archaic Sampi

The Wikipedia has the following information about this codepoint:

Sampi (modern: ϡ; ancient shapes: , ) is an archaic letter of the Greek alphabet. It was used as an addition to the classical 24-letter alphabet in some eastern Ionic dialects of ancient Greek in the 6th and 5th centuries BC, to denote some type of a sibilant sound, probably [ss] or [ts], and was abandoned when the sound disappeared from Greek.

It later remained in use as a numeral symbol for 900 in the alphabetic ("Milesian") system of Greek numerals. Its modern shape, which resembles a π inclining to the right with a longish curved cross-stroke, developed during its use as a numeric symbol in minuscule handwriting of the Byzantine era.

Its current name, sampi, originally probably meant "san pi", i.e. "like a pi", and is also of medieval origin. The letter's original name in antiquity is not known. It has been proposed that sampi was a continuation of the archaic letter san, which was originally shaped like an M and denoted the sound [s] in some other dialects. Besides san, names that have been proposed for sampi include parakyisma and angma, while other historically attested terms for it are enacosis, sincope, and o charaktir.