Okay, even if Bristol cars are going to be very possessive of their 412 light inventory, I can't understand why these lights are so hard to find. Surely there are still quite a number of Opel Rekords of the 1972-77 era still kicking around in driveways and scrapyards especially in Germany, and also South Africa and Iran (pre-revolution) where they were produced as a Chevrolet. I don't have prod numbers for the D series, but nearly 1.5million each of C and E series were produced so it can't be far off this.
Even assuming 95-98% of the cars have been destroyed by now that leaves perhaps 50,000-100,000 still on the roads or a possible 100,000-200,000 headlamps! I saw a mint example for sale a couple of years ago for less than £2000, so a basket-case with complete front lights isn't going to be expensive even if you just want the headlamps. There is the slight issue of what you do with the rest of the car, though.
Just found a brand new one on ebay to labour the point. Okay it's only the left one and it has an indicator light (does the 412 have one?) but is only $99
http://cgi.ebay.com/ebaymotors/OPEL-...Q5fAccessories