Older style fuses with a glass body and metal end caps joined to the fusible link inside, with the number referring to the overall length of the fuse. 30mm fuses are 6.4mm in diameter (1/4″). Generally now used in lower current applications such as audio head units and accessories (cigar lighter socket plugs etc.)
These fuses are normally used in fuse holders that screw or twist-lock together. The 30mm & 32mm fuses are generally interchangeable because the springs in the holders accommodate the small difference in length.