ANOTHER CHALLENGE (WILL THEY NEVER END?)

Here's a fiendish cadet challenge.  This is a test of general knowledge, no logic or math.  No trick to it, either.  Here it is:

What is in the third caster?  (Or, more accurately, what went in the third caster?)

This challenge will remain open until solved.  Good luck.

Here's a hint.  First, look up the definition of caster so that you know what you're looking for.  Next, study the question carefully.  Note that I said "Or, more accurately, what went in the third caster?"  You may take this to mean that the third caster is no longer is common use.  This, combined with the rest of the question, should point you in the right direction.  This challenge will remain open until solved.  The credited solution will provide an explanation.