English: Maybach HL120 TRM V-12 gasoline engine (r.) and Maybach Variorex SRG 328 145 pre-selector gearbox (l.), with 10 forward and 1 reverse (10+1) gears. This transmission was fitted to Panzer III tanks, Ausf. E, F, & G only. Recurring problems with this mechanically complex gearbox by the autumn of 1939 led to its replacement with the 6+1 Zahnradfabrik (ZF) 'Aphon' SSG 77 manual synchro transmission in all the later Panzer IIIs, Ausf. H to N, although the manufacturing difficulties with the Variorex were resolved by early 1941. Source: Jentz & Doyle (2007): Panzer III - Ausf. E, F, G. H. Panzer Tracts No. 3-02.

Legend, clockwise from top right: Rückschlagventil: Non-return valve (the air compressor is directly below). Unterdruckbehälter: vacuum tank. Unterdruckleiter: vacuum pipe. Gelenkwelle: drive shaft. Kupplung: main clutch. Auslöseventil: release valve. Getriebe: gearbox. Fußkupplung: clutch pedal. Vorwählhebel: pre-selector lever. Fahrtrichtungshebel: direction lever (V=forwards, R=reverse). Gasgeber: throttle sensor(?) (There are also acceleration and braking clutches inside the gearbox.)
NB The air inlet of the compressor is connected to the system, not the outlet: the compressor works "in reverse" to create a vacuum. Inside the gearbox there are vacuum-actuated pistons: these move dog clutches, which select the required gear ratio.