LittelFuses Series 285 AUDIO/MEDICAL Quality
I service some of these, and yes there are fast blow and slow blow fuses that have thicker stronger glass tubes, when they blow they don’t shatter and send shards of glass everywhere, this is why they are used, but they use the same internal fuse element.
So if you fusers believe the thicker glass does some snake oil effect for the sound, in Aussie slang all I can say is, goodonya!
Cheers George