I don't know the date of it's manufacture, but I have closely examined one of my Simpson 260s (series 2). There is a separate compartment for the fuses and batteries. This compartment is molded of the same material as the main case of the meter, I would guess it is Bakelite. Both fuses are mounted in molded, vertical holes in the case. The 2A fuse is completely enclosed and the 1A is enclosed for about 75% of it's circumference but a slot in it would allow any fragments to enter the rest of the fuse and battery compartment.
And Simpson was very careful to state that a particular 1A fuse was to be used. They provided both manufacturer and part number. I have that, exact fuse in my 260s and can tell you that, while they are glass fuses, they are the 1.25"/31mm long AG3s which have at least 3/4" between the metal end caps. They also have a very small element that must limit the amount of gas or liquid metal that would fly about. I don't know what, if any amount of energy it would take to cause the glass outer envelope to be ruptured.
When you add the shielding from the case itself, it is hard to imagine that even with the largest possible "explosion" that the fragments of glass and metal would do any damage outside of the compartment. The inductance of the test leads themselves may be the limiting factor in any test or accident scenario.
To be honest, I can't recall any other meter that I have used or seen that had this much protection against an uncontrolled arc. My more modern (digital) meters and even other meters of the age of the 260s all have had far less insulation and just plain MASS between the input circuitry and the user. In many cases these meters that boast Cat III and Cat IV protection will have a thin plastic outer case and some internal precautions like slots in the PC board.
Again, just some observations I have made. It seems to me that, properly designed, the two fuse protection is fairly good. I do not know how these older meters would measure in an actual test. But then, I do not know just how well the testing procedure for these Cats have been designed.
I do know if I was taking measurements at a service entrance with heavy current capability I would feel safer with one of my Simpson 260s than with many of those Flutes with the Cat ratings. But that's just me. And I do know, from personal experience, that the 260 is not much protection from 40KV. But then, I seriously doubt that the Flutes would be either. Voltage and current protection are two different things.
It was a short-term thing in the 1980's - multimeters using two fuses in series, the cheap one and the expensive one. Back then IEC 1010 was still in its infancy.
The fuse (blow) test that UL will do, it's very high energy to flush out a fuse that will explode due to the arc/heat (glass housing), or a fuse that catches fire or arcs to nearby stuff, or a fuse that never actually clears 100%, among many other things they test for.
So a cheap fuse will not clear properly with high voltage/energy, prone to explode and spray glass everywhere, arc that can move to nearby components inside the DMM or just across the fuse holder clips even, in the case of small fuses like 5x20mm.
These (big 38mm) DMM fuses have sand inside to quench the arc, Melamine high-temp housing- that is part of their extra cost.