Reply to "Clutch Slave Replacement"

Your first question" missing snap-ring. These are high-tensile-steel and rust easily. Eventually, they break in two. The fact that rust & corrosion is so heavy on the bracket leads me to believe the ring rusted to failure. Or, someone may have simply lost it. The upper one is less important than the lower due to the way the assembly is held in the bracket.
#2- the lack of threads is because no one has yet changed the slave cylinder bellcrank lever one (1) spline further toward the back of the car, thus adjusting the cylinder. Not all the linkage adjustment is in the rod length. Your long-throw may need two splines backward. You'll just have to try.