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You could try a Google search instead, for example if you put this into the Google prompt, your thread is the 1st hit.
nonequivalent extensions site:mathoverflow.net
Only searching for extensions results in too many hits to be useful, it seems.
generally, the search feature in the software is very poor. I recommend that you use google, e.g. extensions site:mathoverflow.net. The page you're looking for turns up on the seventh page of results, at least for me.