You could try to find out if Conrad rebranded it at some point, they sometimes have schematics and calibration instructions available. They have sold a bunch of Metex products under their own brand 'Voltcraft'. I believe these days it tends to be Uni-T. Once you've found an article number, you should be able to find the manual on the
English or
German website. Their German website is the oldest and usually the most complete.
Failing this, the only option I see is trial and error. Only four trimmers, and a limited number of options. My guess is one for DC voltage gain, one for AC voltage gain, maybe one for DC offset or for the lowest DC range.