It has been years since I've worked on an early-Cj5, but the master included a small built-on reservoir. They had an odd bolt on the top that you had to remove to top-off.
My assumption is the small reservoir is due to space issues. The heater-core sits where most cars have the master/reservoir. So on the early CJ5s, the master is down low, attached to the frame.
On a related note, I've always had great success using the gravity method... but with such a low master, I wonder if thats even possible?
Edit: Here is a picture. Does yours look like this?