Early P-08's did not have the three longitudinal grooves along the firing pins that some say were added to keep the FP from becoming a piston when a primer is pierced, and one of the Scandinavian country's military which purchased early P-08's drilled a small hole from the bottom up into the firing pin tunnel to just behind the transition from the small hole to the larger one for the same purpose.
I've read that when a primer is pierced in one of those early P-08's the firing pin is slammed back so violently that it fractures the little locking surfaces that holds the retainer in place, this sounds like what your's had happen, since there is nothing else going on that exerts that kind of force on the retainer.
But I'm not a gunsmith and am quite often wrong about many things.
