The lower slot in a B&W or an early G4 intended for a Zip drive. If you can get an internal floppy that has the same dimensions, there is no reason it would not fit. You might have trouble finding a faceplate, however.
G3 and G4 powermacs upto Quicksilver have solderpads on the mother board for an 20 pin connector that would seem to be an internal floppy interface. While the low level hardware to support a floppy seems to exist it is doubtful that a floppy driver was ever written to support it. This conjecture is based on the knowledge that these same pads were on the first iMacs and could be uses to support a floppy drive, but remember an iMac is not a PowerMac so while the modification worked on the one it may not work on the other.
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