Not to put a damper on the "hand-crafted" W113's business, but when I was in college taking a Quality class, we visited a local Ford asm plant (Milpitas, CA) in '68? or '70? every day for a week to "evaluate" the quality control conditions (or lack thereof) over each stage of assembly.
There were no "robots" .... everything was being put together "by hand"... guy's/gal's with pneumatic tools and pneumatic lifter's & chain hoists, etc. along the entire assembly line, along side and under it. Body seams were all hand leaded and ground / sanded ready for paint. Painting was by guy's in space-suits...no automated painting robots. At the end of the asm line, a swarm of "fix-it-uppers" (we called them) did the pre-quality inspection fixes... swarming over & under the car (car over a pit), each person with a specialty, "fixed" by adjustments all things ... body panel warps, paint blemishes were taken over to the side and "blended out" on the spot (in a kind of make-shift paint booth) with another person using a big hot air gun on wheels to "dry" the freshly painted blend. There were stacks of spare parts --- so that if a seat didn't work properly, they removed the malfunctioning seat asm, and replaced it on the spot. I got to "evaluate" the underbody "fix-it-upper" section one shift.. they mostly pushed and yanked the exhaust into position, replaced missing nuts, bolts, and screws on the chassis to suspension and similar attachments, looked for leaks at the brakeline-brake joints, etc... tightening the connectors or replacing the brakeline hose when that didn't work, etc.
When the "fix-it-uppers" were done, a lead "fix-it-upper" passed judgement and if acceptable, the car moved on to the quality control inspectors --- there were only a few of these for each car... they'ed start the engine, put it in reverse, back up 5 feet, then 1st and move the car forward while the quality inspectors went over the car, honking horns, turning lights on and off, exercising signal and brake lights. When it was "passed", they gunned the car into 2nd, screeching tires and ran the car outside to park it in the huge production lot for eventual loading onto rail cars.
So... in late '60's & early '70's Ford was "hand building" the cars... there were the usual chalk marks and indelible pen marks designating what options were supposed to go on each car.. upholstery type, seat types, engine type, tranny type, etc. ,and marks by asm's (indicator's of which assembler had done the work (this was so quality inspections could feed-back and track quality by assembly person), marks by quality control showing such-&-such had been inspected (and by whom), etc.
The only difference may be that there was no hand-crafting of upholstery... dashs were already sub-assembled by a vendor, as were seats with seat coverings. But, chrome trim was "hand" adjusted by the swarm of "fix-it-uppers", bumpers re-aligned by same (kicking them into position wasn't uncommon at all), small dents removed by hand on the spot (just like the "dentless painters" do today... they probably learned their skill at a car factory)... extraneous body scuffs were buffed out, etc.
Those were the days when Quality wasn't built into the cars, but rather was inspected into them at the end.
I've never bought an American car (other than my '65 Chevy C20 pickup) because of that experience.
I'm sure the German asm lines and assemblers were more conciencious --- my time and experience in Germany being my basis... and more-so at MB assembly plants than perhaps at others, since working on the asm lines for MB was the crem-de-la-crem assembly line job if you did your job "correctly". The differnce between the MB plants and GM's at the time may have simply been "pride of workmanship".
The real differentiating factor though wasn't in assembly, but in quality of engineering design for the parts that were being assembled.