actually, i just found some info on them on a german website, but i translated it using babel fish... it says:
it says they were made in 1977 i think, and it says that this model has... get this... 600 OMHS!!!
also, the dude who wrote it says something about AKG with the cans, not so sure what it is...
unfortunately, i cant understand a lot of it even translated cuz babelfish sucks.http://www.elmulab.de/wiki/Sennheiser+HD+420
is the site if anyone's interested.
also, i found a few professional recording studios use these cans, obviously there pretty good then. now, to find an good amp that'll be able to drive these things...