I thought I would post the following from Dave Gallon who many know. This is in regards to whether the W111 coupes and cabs REALLY had the US license plate lights with the bulb holder screw at the bottom. There have been a few books that approach the topic, but Dave provides a perfect explanation:
In the 1950s, there was a "186" license plate light. It was mounted on the trunk lid and so it was reasonable that the bulb access and drain would both be on the bottom.
When the 220SEb coupe/cabriolet US version was configured, they used this same license plate light with an additional base. The only problem is that it was now mounted on the rear body panel so the bulb access on the bottom was really inconvenient. Nevertheless, DBAG continued this arrangement through the end of production on the 111 coupe/cabriolet in 1971. (To be clear, it was nearly impossible to replace the bulb without removing the entire license plate assembly from the car.)
With the introduction of the 108 sedans, DBAG produced a new US version license plate light, the "108" light, with the bulb access on the top. (The bulb access is on the top, the drain is on the bottom.) This is a much better design.
The conclusion is that the "186" light was used through the end of 111 coupe/cabriolet production and then shortly thereafter discontinued as a spare part. The 108/109 sedan used the "108" light through the production run. The "108" light replaced the "186" light as a spare part in the 1970s. So, if you see a really good US version 111 coupe/cabriolet it will have the bulb access on the bottom and the drain on the bottom. If the license plate lights have been replaced due to collision damage or weathering, they will almost certainly be the "108" lights and who knows how they will be mounted (I've seen every possible combination.)