Ogham as it survives on stone is a twenty letter alphabet in four aicmi of five: Beithe, hUatha, Muine and Ailme, each a family of notch and line strokes cut along the edge of a standing stone, mostly commemorating a name in Primitive Irish. Aicme Forfeda does not belong to that inscriptional layer. It appears only in the later manuscript tradition, chiefly the grammatical tract Auraicept na n-Eces and companion texts such as De dulib feda and In Lebor Ogaim, compiled by Irish scholars working from roughly the seventh to ninth century onward and copied in medieval manuscripts centuries after monumental ogham had gone out of use.
The five forfeda, Eabhadh, Or, Uilleann, Ifin and Emancholl, were devised to cover sounds and diphthongs the original twenty letters did not represent, and in some manuscript traditions to supply a letter for borrowed sounds such as p, which Primitive Irish lacked. Their number is not fixed across sources: some tracts list four, others five, and a few extend the set further. No forfeda letter is confirmed on a surviving ogham stone, which marks the aicme as a literate, scholarly extension of the alphabet rather than a fifth family cut by the same hands that carved the original four.
Sources Auraicept na n-Eces: The Scholars' PrimerGeorge Calder (ed.) with A Guide to OgamDamian McManus