The black link looks like it was spliced in at some point. It's possible whoever did it didn't ensure the link was freely moving. [MENTION=78399]Bill Kapaun[/MENTION]'s advice is sound; flex the chain sideways at that link and it should free up.

If it doesn't, you may be able to replace it with a master link, but it's possible a modern master link might not fit that vintage 1980s SedisSport chain. If that's the case, a new chain, and likely a new cluster will be needed. Try the simple fixes first.