The first production IBM hard disk drive, the 350 disk storage, shipped in 1957 as a component of the IBM 305 RAMAC system. It was approximately the size of two large refrigerators and stored five million six-bit characters (3.75 megabytes)[18] on a stack of 52 disks (100 surfaces used).[27] The 350 had a single arm with two read/write heads, one facing up and the other down, that moved both horizontally between a pair of adjacent platters and vertically from one pair of platters to a second set.[28][29][30] Variants of the IBM 350 were the IBM 355, IBM 7300 and IBM 1405.