The precise subject of the Martyrdom is puzzling...That it is a scene of martyrdom is clear enough, but the identity of the saints is uncertain. Giorgio Vasari referred to the woodcut as a Beheading of Saints Peter and Paul, and this description is generally followed, but it does not fit with Peter's traditional death by inverted crucifixion...A simple explanation for this might be that Parmigianino decided to dispense with the tradition of Peter's crucifixion tradition for an unknown reason, and simply wished to depict in a single space the unorthodox execution of both apostles.

from Coleman, Seminar in Sixteenth and Seventeenth Century: Old Masterworks on Paper (Notre Dame, 2013)