This breaker was replaced after a few years of service installed in an older model zinsco panel.  It was replaced because one pole had tripped and was not able to be re-set while installed/seated in the panel.  Once the breaker was removed from the panel, we found that if you depressed the pole that tripped a bit harder through the range of motion, the breaker properly reset.  This seems to be an design issue with this style breaker and it probably didnt need to be replaced at all.  For what its worth, the replacement from the supply house was already installed on the panel so there was no going back after the new one was installed.  This breaker passes continuity tests just fine.