Voltage regulator probably. If someone upgraded to a newer alternator with an internal regulator, you can probably remove it entirely.
What's behind the cover if you remove it? I can't see any reason for a removable cover if it's a relay. A regulator (old school) usually has some sort of adjustment though.
1996 FZJ80 Born: 08/95. Died 11/11(est) @162K.
Cause of Death: Oil Starvation - Front crank bolt loose.
In the "operating room" since my purchase date of 6/8/12.
Got the block back from the machine shop 8/27/12.
First start 01/26/13. Running since!