Guten Morgen Bruce,
Found the manual (in German) on RC Groups site.
First off-
Lead #2 is labelled 'Blitzerausgang' = Flasher output.
Seems to be intended for a flashing light, i.e. police or emergency service boat.
Probably a mini light bulb back then as LEDs were still exotica and expensive.
It has nothing to do with RPM variation of the diesel sound. So not needed unless you want to go flashing😮🙈
Unfortunately there is absolutely no reference to Lead 2 in the manual!🤔

"Should the sound of the RPM speed up when you advance the throttle?"
Yes😉 The module takes it's input for this directly from the terminals of the motor, i.e. voltage being applied via the ESC. So you need a mating connector for lead 3 wired to the motor - brushed only I suspect. Polarity is not important - logical since the motor can run both forward and backward.😉

"adjustment switches" these are actually adjustment potentiometers, variable resistors (known as pots).
Diesel Drehzahl (RPM) - is used to set the diesel sound RPM to your taste when the motor is running full chat on a bench test.
Diesel Lautstarke (volume) - is used to set the max volume of the diesel sound.
Gesamt-lautstarke (total volume) - is used to set the total output volume , i.e. when all available sounds (diesel, horn, ship's bell, wave slap🤔) are in use.
NOTE: there are warnings about setting this too high as it risks damage to loudspeaker and/or the module amplifier. More about that later.
For now; you need a minimum 4Watt loudspeaker.

Have looked at the module specs ... but more tomorrow as it's now 2am here and I'm starting to nod off😴💤💤
Cheers, Doug😎

https://www.rcgroups.com/forums/thumbgallery.php?t=1771514&do=threadgallery&type=img&group=none&starter=no