The photo is not clear enough to show the 15/8, but it is cast into the white ceramic body.. I don't know if it is a fuse rating (15 amp or 8 amp) or whether it has nothing to do with the fuse link installed on the ceramic body. Which brings up the question - if not the fuse rating, how do you know the fuse rating?
I have a collection of these fuses from fifty years ago when I had Beetles. Some of the fuses have rose coloured ceramic bodies, may be the body colour is tied to the fuse rating.
The fuses come in handy when I work on my friends' air cooled BMW motorcycles.