OK, now I get it. See the 2 points where it's screwed/pushed up onto to your chassis ?
Looking at your picture, they are top left and right of the white plastic mount, on the under side of the chassis you have two inner kinda bent beveled "cuts" into which screws or pushed in "mushrooms" hold the white plastic mount to the underside of the chassis.
Your problem is that they were screwed or pushed in just before the alloy top plate was glued on the top side of your chassis... To undo it from "below" is barely impossible.
I believe that to unsolder those wires is the cleanest thing you could do to remove the tonearm without damaging permanently any other parts.
tweaked Thorens TD145+'Eagle & RoadRunner'
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