Unfortunately, IPS never provides older versions for download, not even for a simple comparison. I have a folder with all versions myself, but I'm not allowed to share the package/files. 🤷♂️
A way to work around the sharing problem would be to hire someone with access to the 4.3.6 package and have them compare the files for you.
Here are a few considerations:
While the 2.x and 3.x versions used to require extensive file edits to install modifications, starting with version 4, there were no file edits at all. If you have third-party modifications installed (applications and/or plugins), they'll show up in your ACP. It is very rare for someone to edit the core/default files of the 4.x version.
I recall that "serviceworker" file had some changes/fixes at some point, but I don't recall which version exactly. However, it might simply be a bug that's fixed in a later version. 🤔
Since you mentioned making a test copy, I would simply try upgrading that copy to the latest 4.7.19 version and see if the issue is fixed.
The summary is: you most likely don't have any file edits.