It looks like you have a commercial Mauser 98-type receiver that was badly corroded - as evidenced by the condition of the bottom, which normally should be almost as smooth as the top.
More bad news is that it could be a cast receiver, and not a forged unit.
A build on it, besides any safety issues, would be (as posted) economically unfeasable (aka: a money pit)- BUT, if you have deep pockets and are determined to use it for a build, should definitely have it professionally tested for hardness & magnafluxed for any internal flaws.
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