FYI
I got a HP 3478a in without a mains fuse holder cap and could not find any info on a current replacement part
figured out the 3457a used the same fuse holder cap .. not that provide much more info
But found and got on ebay 'Bussmann FBI, Fuse Holder Cap for ¼” x 1-1/4” Fuses' AND IT WORKS!
so I thought I would leave this info for anyone else in a search for replacement for either model