I read the date as "Fer. II. Pasch.", that is the 2nd feastday of Easter -- your interpretation of the Monday following Easter Sunday is valid.
Fer. = ferias (holiday)
Pasch. = Pascha (Easter)
For the numbering, compare the "12 Days of Christmas", which counts Christmas (25 Dec.) as #1 and Epiphany (6 Jan.) as #12.