It appears it came with the the Keysight meter.
What do you mean by "appears"? Did you personally unbox a new/sealed Keysight meter and remove that fuse from it?
I bought the Keysight meter from Keysight, brand new. Yes I unboxed it and proceeded to test it to failure. Very few meters I have looked at came with SIBA branded fuses. Most that did, used the small body style. The Keysight meter I bought did have a 10A large body and I had pulled the fuses when I scrapped the meter. The pulled fuses get tossed into a bag. There is only one 10A SIBA fuse in the bag. But, I've ran many meters over the last couple of years and it's possible there was some other meter that came with SIBA branded fuses that I scrapped. So I use the word appears because there is a chance that it came from something else. I'm not sure what I would have done with the fuse from the Keysight though.
I had made a series of videos where I modified a UT61E. One of the things I had done was to replace the small fuse with the larger one using this SIBA fuse. However, later changed out the shunt to allow using the meter at 20A and replaced the 10A fuse and back into the bag it went. I can't seem to find any other videos showing the 10A fuse and am fairly confident of it's history.
https://youtu.be/d6LTsaOqk30?list=PLZSS2ajxhiQDrk4o1Y45auwK7LomjnNBU&t=274
Looking at the pictures I have taken over the years, the Gossen and CEM meters used the large SIBA 10A fuse. However the Keysight meter does NOT!!!!
It's a good thing I take pictures. I forgot that meter does not even have current.
I may need to retire from meter testing.
I still have these meters and could take high resolution pictures of each fuse if you like. You would expect a Gossen meter would have a genuine SIBA fuse. See attached.
So, now that we know the fuse in question did not come from Keysight, where did this fuse come from? I think there are two possible answers. I had ran another CEM at one time and I think there was a Klein meter that may have had a large fuse as well. I did not take pictures of these meters and would need to see if the videos show anything.
The ones I had purchased that I suspected were counterfeit were clearly marked different than the ones supplied with the meters. They also behaved different when I compared the two.