Nice Avatar Simon! That's an oldie but goodie!
Yes, it is a supplemental fuse panel, but not like what you would think, as in secondary surge protection. It just was used to split out the high-amperage outputs of the power supply into groups of line card slots.
The 512 KSU that ITT made was a variation of a Western Electric 584 panel on a rack with a cover. It wasn't truly a 584 panel though, because 584's had lots of fuses to localize power distribution to the individual cards or groups of cards. While the 512 was still a 13-line KSU, it lacked this individual localized fusing of high-current outputs, like lamp power.
Even with small phone stuff, those 1A2 power supplies threw out a lot of amperage, sometimes 10 amps. A short under a single 10 amp fuse would have certainly caused a risk of fire. I mean, 18 gauge wire is rated to carry that amperage, but when you get down to 26 gauge line cord conductors, it could get ugly.
Those fuses were just there to break the 10 amp output into more managable 2.5 amp circuits. I am sure it was a UL/CSA requirement.
