Sorry to hear that you're epically frustrated.
I've spent 3 hours today setting up older Zen Cart versions to inspect this very situation for you.
I've compared code between versions, and discovered that all the code that draws the manufacturers_select sidebox is 95% the same between 1.5.1 and 1.5.7.
I've also compared the functions that the sidebox uses to generate related HTML are 80% the same.
In both cases, switching the 1.5.7 code back to the 1.5.1 code resulted in the SAME output as the 1.5.7 template.
I set up stores with custom ancient PHP and MySQL versions to attempt to simulate the problem, and in every case the 1.5.1 code showed the same kind of dropdown as the 1.5.7 code.
So I'm pretty confident that the problem is not in original Zen Cart code, and that it's specific to a customization you've made unique for your site.
Obviously one customization you've made is that the "manufacturers" is called "authors". Was that just a text change for the title? Or was it actually an "authors" sidebox that was created as a clone from the one for manufacturers before customizing its functionality?
Are you using custom product-types for books? Perhaps something in that is what was handling things on the old site, but hasn't been moved into the new one yet thus the display doesn't reflect it yet?
You mentioned "bespoke changes", but nowhere in this discussion have you outlined any of those bespoke ways in which your 1.5.1 site was customized. I clicked around some of your other posts, but this info isn't in those and I got tired of guessing.
You also haven't provided a link to the 1.5.1 site so some of the bespoke changes could be explored in lieu of you explaining them. Looking at the actual HTML of the sidebox could reveal some clues. Maybe the site doesn't currently exist online, but if it does, it would be helpful.
After tinkering for hours, trying to guess at what info you haven't provided, I'm done.
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