It may be that the original importer took a "once in a lifetime" exemption for the car, and that would explain some of the lack of paperwork. In 1980 I believe there were emissions standards that were being met by retarding the spark to lower temperatures in the engine so you formed fewer oxides of nitrogen, and an air pump and exhaust gas recirculation. I do not think catalytic converters were the norm at that time, so you may not have needed an Oxygen sensor and fuel pressure control valve. At any rate, if the car is here and registered I would leave a sleeping dog alone.
Many grey market cars have the original bumpers, but with steel structure added behind the chromed and shaped exterior to meet the standard of the day for crash resistance. You can check by looking behind the bumpers and seeing if there is a strip of flatbar going from mount to mount across the front and rear bumpers.
Pfizer has two locations here now, one in Groton which is their older facility that included R&D, product development and production. The new location took the R&D facility into New London to make room for the other activities in Groton, I believe. Pfizer is a major employer in the area, and the newer, better, and bigger R&D facility brought a lot of new, young, and reasonably affluent professionals to the area, driving up real estate values. The area is still pretty rural when you get 10 minutes out of New London, and the smaller towns are run by old Yankees who like things the way they used to be. Old Lyme is a great town to raise kids, but don't expect DSL or cell phones to work here!
Good luck with fixing your clutch if that is next. I would agree this is no VW, and a proper lift with a stand for the transmission would make the job much safer. I have never done this on one of these beasts, and like you have done it on a VW bus and bug in the old days as a matter of routine. Once again, Good Luck! 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)