Hahn & Kolb were a large German machine-tool organisation not dissimilar in many respects to the English Herbert Company. In addition to manufacturing their own machines they also factored and distributed those from many other companies - in some cases forging so close an alliance with them that the machines lost their original identity and were badged Hahn & Kolb instead.
Built in 1937,  the machine illustrated below was based on a design that was to become popular throughout Europe* - and as originally developed though the contemporary  Thiel 58, Maho SK250 and Deckel FP1 types. A rare machine, and almost certainly sold only in continental Europe, the early Variomat was fitted with a 2-speed, 3-phase (1 kW, 1500 r.p.m.) Siemens motor that powered two infinitely-variable-speed "gearbox" units by PIV (a maker still in existence) - one connected to the spindle and the other to the table drive. In the diagram below can be seen two sets of 4 conical expanding and contracting plates marked REGELTRIEB - these provided the alteration in ratio as they opened and closed.  Whilst for lower power applications an ordinary "rubber and canvas" belt is used on this sort of drive this version employed conical plates with "teeth" - the belt being replaced by a special steel chain that could mate with the pulley indentations. The spindle variator worked though a safety-overload clutch, and completely integrated into the body of the machine; that for the table was similarly clutch protected though easily dismounted for maintenance. The system gave two speed ranges: low from 50 to 260 r.p.m. through a reduction gearbox and high from 250 to 1200 r.p.m in direct drive. A dial on the side of the column showed the various setting with the upper window displaying the cutting speed in m/min (depending on the milling cutter diameter - the words Schnittgeschwindigkeit and Fraser D can be made out) and a lower aperture giving spindle revolutions per minute (umdrehungen). A separate window displayed the table-feed rates - these varying from 10 mm/minute to about 120 mm/minute. Interestingly, the motor was switched on by a lever (Motor Ein) not a push-button, with a second lever (Getriebe Ein) to engage the spindle-variator clutch. The latter was spring loaded, and required some force to move, the clutch engaging gradually across the arc of movement - in the manner of a car or motorcycle type. There was also an Einrichten position by which means the engagement could be "tuned". Once running, the setting of each variator was independently altered  by means of separate 3-spoke capstan handles on the right-hand face of the main column.
Apart from the ingenious drive system the miller was conventional for its type and of the standard German design, a Universalwerkzeugfräs-und-Bohrmaschine (all-purpose tool milling and drill press). It used a vertical head that could be driven across the top of the main column (by a handwheel working through reduction gearing) to provide the in and out feeds. The knee, which moved from side to side and vertically, was fitted with a vertical T-slotted surface able to mount a number of different tables - plain, plain-tilting and compound swivelling. Control of the table's movements was by the popular joy-stick method that, as on similar models, may well have been able to engage (deliberately) both vertical and lateral feeds simultaneously.
Fitted with a  No. 4 Morse taper spindle running in a large front bearing of bronze, and a rear roller bearing backed by a separate thrust race the vertical head was a substantial affair. However, unlike most competing machines, the head on the Hahn & Kolb did not have separate upper and lower parts where the top could be removed and replaced by a different type - noticeably one to support an overarm to convert the miller into a horizontal model. This latter conversion was achieved instead by replacing the just the end of the head with a boss - into which were socketed two steel bars that acted to hold the drop bracket. Almost certainly other types of head must have been offered - including self-contained high-speed models - for it is unlikely that the makers would have so limited the usefulness of the machine by not doing so. The Variomat had a long production run, from the 1930s until the early 1970s, when competition from Maho and Deckel finally saw it off.
If you have a Hahn & Kolb Variomat the writer would be pleased to hear from you.
*Other makes of miller arranged along the same lines as the Variomat include:
Austria, Emco (Model F3)
Belgium by S.A.B.C.A. (Model JRC-2)
Czechoslovakia by TOS (Modesl FN22, 32 & 40 Optic)
Denmark or Spain by Metba (Models MB-0, MB-1, MB-2, MB-3 and MB-4)
England by Alexander ("Master Toolmaker") and the Ajax "00", an import of uncertain origin Germany by several companies including: Macmon (Models M-100 & M-200 - though these were actually manufactured by Prvomajska ; Maho (many models over several decades), Thiel (Models 58, 158 and 159), Hermle (Models UWF-700 and UWF-700-PH), Rumag (Models RW-416 and RW-416-VG), SHW (Schwabische Huttenwerke) Models UF1, UF2 and UF3 and Hahn & Kolb with their pre-WW2 Variomat model
Italy by  C.B.Ferrari (Models M1R & M2R) and Bandini (Model FA-1/CB)
Japan  by Riken (models RTM2 and RTM3)
Poland (as the "Polamco" Models FNC25, FND-25 and FND-32 by Fabryka Obrabiarek Precyzyinych)
Switzerland by Aciera (Models F1, F2, F3, F4 and F5), Schaublin (Model 13), Mikron (Models WF2/3S, WF3S, WF-3-DCM & WF-2/3-DCM), Christen (Type U-O and U-1) and Hispano-Suiza S.A. (Model HSS-143)
In the former Yugoslavia by Prvomajska (in Zagreb with Models ALG-100 and ALG200)
- and also by  Sinn (Models MS2D & MS4D), Ruhla and "Comet" (Model X8130, imported to the UK in the 1970s by TI Comet). At least five Chinese versions have also been made, including one from the Beijing Instrument Machine Tool Works.