Brokenrobot's derailleurs are 1st generation 600 models, which were introduced for the 1976 model year. They were superceded in 1978 with the introduction of the 600EX (arabesque group). However, they did remain in the catalogs until at least 1982, for manufacturers and cyclists who wanted a less expensive alternative to 600EX. However, in this case, the front derailleur has a Uniglide stamping. I believe, the introduction for Uniglide was 1977, so the at least the front derailleur would appear to be from 1977 or more recent.
The part numbers for these derailleurs are RD-6100 and FD-6100. The German site does show them for 1982 as Luker states, however, it does not contain any earlier entries, regardless of the components. Obviously, the compiler did not have those catalogs and 1982 is the effective starting point for this website. So we can't say they got anything wrong, just that hey don't have all the data.
In fact, everyone's comments are correct, though the story was somewhat incomplete. ChrisLite was right about the 1977 date code and they would have still been current for the 1978 book that Alanbikehouston saw them in. They were also still in manufacture for 1982, as Luker commented. The bottom line is that these components were available from 1976-1982 and for part of their life span were being produced concurrently with 600EX and 600AX.
As for the Shimano date codes, I have yet to run into any fit problems, provided I consider the circumstances. Components can often sit on LBS shelves for a number of years, so even if you by them "new", they could be several years old. This can even sometimes happen with manufacturers who have poor inventory control and push the remaining stock to the back of the shelf when a new supply arrives. In such cases, the parts can get pushed back repeatedly and it can take a long time for the old parts to surface. Secondly, big manufacturers like Shimano have to start manufacturing components a year in advance, to have the necessary stock for the manufacturers and LBS. So even if you buy the "just introduced" derailleur, it could have a date code from the previous year.