So far it has not been clear whether you are aware of whether this file was originally generated on Orcad or some older version of Proteus. In cases like this, instead of making blind attempts, the best thing to do is try to open the file directly in a text editor and look for some textual indication in some segment of its content for keywords "orcad" or "proteus", as well as their versions.
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Actually, taking a look here at the contents of 2 DSN files, either from Orcad and Proteus, I could find the "Orcad" keyword for the first one, but only the "ISIS" keyword for the second cad.