Trying to find a compatible OS for an old computer

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http://www.unix.com – Hello A friend and I recently got our hands on an old computer (a rainbow '83 I think), cleaned it, etc... And now we're trying to get the thing running. The problem is that the included OS is badly documented, so I thought of installing an old version of UNIX on it. So the questions are: Is that computer compatible with a unix os? If it is, where can I find old versions of unix, preferably before 1983? (HowTos)