I discovered that the coil has expanded at one end and leaked some of its oil out of the case. I suspect this was because it was very hot at some point. I have a Mallory system that replaced the points and changed out the coil to what was recommended for the system. Nothing real fancy here with the ignition system just swapped out the points for this. I hadn't noticed any change in the driving of the car... just surprised to see that happen. Thoughts on why this occurred or what I can do to prevent this from happening again?

Use gloves to clean it up. That stuff is nasty.

It shouldn't happen with the Mallory, but I'm pretty sure back in the day with points if you left your ignition on with the car off there was a chance of over saturation of the coil which would lead to it getting hot.

If power is left on to the coil it is essentially just a 30 watt heater in a small package. It will get hotter & hotter until the battery runs down.

Was the battery dead after the incident?

I used to have a Accel super coil on mine once... left the key on with engine off while tuning it up, and the coil exploded. oil filled coil. big mess in the engine compartment.