The item is a circuit breaker as John said. It is probably a self-resetting breaker. I would check the amperage rating so you know what is considered an overload by the original installer. If you are increasing the output of the alternator and you are increasing the carrying capacity of the wiring; it stands to reason the CB might be too low a rating and cause problems of its own. Never hurts to check!!
Old trucks and old trailers . . . a comfortable combination!
A-6E All Weather Attack Driver, BUFF
USN Aircraft Maintenance Officer, Ret.
'91 Suburban R-2500 w/BBC
'78 Honda GL-1000
'72 Triumph T-100R Daytona