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These days, most people think that the floppy disk drive evolved very logically from 8", through 5 1/4", to today's 3 1/2" standard. Not so! Before 3 1/2" gained dominance, various other formats battled it out.
Here's one of those forgotten formats - the Amdek 3" diskette drive. That's not a typo, it's just 3", not 3 1/2".
This model of Amdisk 1 is for an Apple II.