More than likely there was a different distributor used in those years and Pertronix doesn’t support that distributor.
Which means switch to a later distributor- 1960- up or see who else used that distributor. Maybe pertronix doesn’t know it’s the same.
Wasn’t 60 the first year of the alternator instead of a generator? And there was the 6 volt to 12 volt switch during the 50’s too. That could also be a contributing factor.
You may have to convert to an alternator and then use the later distributor.
I received a call today from a 59 Plymouth owner-----22.000 miles on a flathead 6----does anyone know if they are making a petronix ignitor for this engine ?-----thanks-----Johnnie Slayton