If your car was legally modified to meet the EPA and DOT regulations, you should have a nameplate in the driver door jamb that identifies the importer of record. It should list the address as well. It may list your name or the original owner in this space, which won't do you much good.
In 1986 a Euro version of the US emissions control system was made available for most Mercedes-Benz autos as an option. It made bringing the car into the US much easier, but that is not why it was offered.
Apparently the Green Party found the fact that the cars were for sale in the US with superior levels of environmental friendliness than were available in the Fatherland, and this was publicized as a good reason not to trust the auto companies and impose speed limits. In the end, the systems were made available, as well as unleaded fuel (if your car came from the factory with a cat installed, you might see a sticker that is a green background with white or silver letters that says "Bleifrei" or unleaded under the filler cap) in a negotiated solution to imposing speed limits on all the Autobahns.
A factory installation meant for the Fatherland will also include a round, black plastic fitting, about one inch or so in diameter with green lettering on the driver's side fender with wire running to it. It should have a letter "N" on the top one one side and about half way around to the other side a a letter "S" with tick marks between them. This thing is to adjust the ignition timing for the gasoline you are using, "S" equals "Super" and "N" equals "Normal" (high octane and regular, respectively).
Someone should have records of who performed the modifications on the car, as the door beams and bumper mods, as well as the lights, speedo and CHMSL had to be added regardless. These were not made options on MB's (the door beams and bumpers, I believe however, became standard on Porsches around this time due to an accident by a factory driver on the proving grounds that resulted in injuries that might have been avoided with a US model).
A very detailed report was submitted by the agent selected to handle the modifications to both the EPA and the DOT. These reports may or may not be available for you to copy through the government. I doubt anybody that did the modifications is still in business today though, as when the dollar value dropped in the later 1980's the cost incentive to play in the grey market vanished, and so did the large number of people playing.
Hope this helps, Jim
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)