Bug 29173 implemented search pre-fill for the Replace via Z3950 search form from the authority detail page, but it did so in a very different, and MARC21 specific, way from the same button on the 'Edit authority' page. On the edit authority page we extract the pre-fill terms from the current page and pass them as URI parameters to the search pop-over form. I think we could take a similar approach on the detail display page and perhaps even make the feature work for both MARC21 and UNIMARC in this way?
I don't see the difference, it looks like both pass the authid and pull the terms from marc, and accounts for UNIMARC, maybe another bug fixed it? See also bug 33445