Actually

they are not from the latin alphabet. Those are upper case Greek Letters. They just look like latin alphabet letters cus they have the same shape.
Just like X (eks) looks like X (chi) and P (pe) looks like P (pi) . They look different as lower calse letters.
IHSOYS is Jesus in upper case greek. In greek as in latin the IHS would stay fixed and the endings would change to indicate the part of speach (subject, direct object, object of preposition, indirect object, etc)