The origin of the prominent 'knee' in the cosmic ray energy spectrum at an energy of several PeV is still uncertain. A recent mechanism has shown promise, however; this involves particles from a very young pulsar interacting with the radiation field from its associated very young supernova remnant. The ensuing nuclear reaction of the particles with the photons by way of e super(+ e) super(-) production then causes the characteristic knee. In an earlier paper we argued that the mechanism would imply only one source of a very rare type - if it were to explain the spectral shape. Here we examine the mechanism in more detail and conclude that for even a single source to work its characteristics would need to be so unusual that the mechanism would not be possible for any known type of pulsarsupernova combination.