Pubul57,
The ones I have are Bussmann's ABC. Below are the specs for the 4Amp fuse I use for the mid-range driver that I got from Master-Carr. It doesn't mention whether it is sand-filled or not. I also have the 2.5Amp fuse for the tweeter, and that one is also sand-filled.
Part Number: 71385K29
UL Specification :: Listed
Bussmann Type :: ABC
Littelfuse Type :: 314
Ferraz Shawmut Type :: GAB
Available Amps :: 4
AC Voltage Rating (VAC) :: 250
Fuse Type :: Fast-Acting Fuse
Ceramic-Tube Fuse Type :: Fast Acting
Fast-Acting Fuse Type :: Ceramic-Tube Fuse
Visual Indicator :: Without Visual Indicator
AC Interrupt-Current Rating :: 10,000 @ 125 VAC
DC Interrupt-Current Rating :: 10,000 @ 125 VDC
Diameter :: 1/4"
Overall Length :: 1-1/4"
Specifications Met :: Underwriters Laboratories (UL)