I've been working on 190SL's 10 years longer than 113's so I know them a little bit. In the picture of the 190, you will see that the engine was installed without the trans but with all manifolds, carbs and the plenum chamber installed. This would speed up the job considerably as it's a chore to install the engine with the trans and one of the other problems is that long header pipe that can only be installed from above. If the engine is installed with the trans, all of the carbs and manifolds have to be off the engine and must be installed afterwards. This would of added considerably to assembly time and they were trying to make money. The factory would of had all of the tools and jigs to make all of this happen easily whereas I have to do all of this the hard way.
It should be noted that this wasn't an ordinary assembly line. It moved very slowly with only about 15 cars rolling out the door every day. The idea that these weren't quality built cars ( as stated by by a poster in the comments section ) is pure horse #&%$ and it really attests to the ignorance some people have about MB after the war.