I asked Baofeng but they said it was a standard auto fuse. But I went to 3 auto stores & tried 3 brands of auto fuses & they were all too thick. The employees confirmed this. The fuse in the adaptor is 0.5 mm thinner than the auto fuses.
The fuse is not 20mm long, it's 30mm long. The 30mm I found were generally 6 mm wide, which are too wide. I checked Newark & Mouser but they just had 6x30, not 5.5x30, which is what I need. I'm beginning to think they don't exist. Even if my adpator is defective, I can't figure out how they found a fuse that will fit. Both I & an employee measured the fuse with a caliper (2 separate calipers) & found the fuse that came with the adaptor is indeed thinner than 6mm.