These are the first generation of 720K floppy disc drives from Vortex. They came in variants having either a single drive ("S" variant) or two drives ("D" variant). The "S" variant (as well as the less expensive "Z" variant) was delivered with the same double-height casing as the "D" variant, so that a second drive could be retrofitted.
The F1-S, F1-D, M1-S, M1-D drives have their own FDC765 in the controller, and this one supports all DS0 up to DS3. The Vortex interface can manage 4 floppy drives in total.
=== F1-S and F1-D (5,25") ===