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.
=== F1-S and F1-D (5,25") ===
The F1-S and F1-D models are based on a single (F1-S) or two (F1-D) BASF 6138 (81690-001) type 5,25" disk drives, i.e. double-sided, 80 tracks per side, 96 tpi density.
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=== M1-S and M1-D (3,5") ===
The M1-S and M1-D models are based on a single (M1-S) or two (M1-D) 3,5" disk drives.