The extra fuse holder may be for a spare fuse.
Or it may be not used
One school of thought is to fuse both positive and negative leades.. Some things do that (Some of my radios) I won't go into why.
Others do not feel the negative needs a fuse.
My thing. If the somewhat rare failure ICOM and Kenwood think my radios need protection against happens.. Well the radios are hundreds to 2,000 dollars.
That switch.a couple bucks.
Home was where I park it. but alas the.
2005 Damon Intruder 377 Alas declared a total loss
after a semi "nicked" it. Still have the radios
Kenwood TS-2000, ICOM ID-5100, ID-51A+2, ID-880 REF030C most times