Manual disc brake master cylinder?

That's actually a great starting point.
See if you can then verify the '80 Aspen MC has the same mounting pattern as your car.
Also some years/models the studs are in the MC, others in the firewall, even though the pattern is the same. They can be pressed out of the MC so don't let that be deciding factor if everything else is the same.
Check the outlet sizes.
A little bit more difficult, but you may be able to verify is the stroke and pushrod. It's probably the same but if you can check it, its one less thing to have as question.

The car is going to be on jack stands for a while so I’ll just pull the old one and take it to the parts store with me to compare them.