Must still be some around Mark, your comments made me wonder. I intended to refit my old fuse box on the 500K, I never liked the single fuse on the 500 so I replaced the loom with a 550F one which as you know has 3 fuses, the fuse box is still in really good condition and as an added bonus the 500 has a spare fuse holder in the tooltray, so I can carry quite a few spares, just in case. With this in mind I bought a few bags of fuses a few years ago, your comments on how the SFE fuses made me wonder excatly what I'd bought so I went to check. 2 bags were the AGC/AGX type, really thin wire running between the end caps, however 2 bags were exactly like you describe the SFE fuses, rather thick strip at the end caps narrowing to a much thinner strip in the middle, a strip not a wire. Bought mine off Ebay so might have just been lucky or there may still be some around if you look hard enough.
The 2 bags I bought were 15a and 7.5a BTW.
As it turns out I bought the blade type fuse box off you Mark, so these are surplus at the moment, not 100% sure which way I'll go yet TBH.