Then what's your theory as of why the 2013 10.9 Finder recognizes the 1995-era System 7 version of Finder as "its own"?
It's probably because the System 7 Finder has the "system" bit set in its Finder-flags. This was used to prevent accidental deletion, among other things. That is, if you dragged a system file to the Trash, the Finder would prohibit it, or maybe confirm with an OK/Cancel dialog (I've forgotten exactly which).
The system-bit is a single bit in a flags field of the FinderInfo metadata of a file (bozo-bit and custom-icon-bit are two others). The FinderInfo itself is a 32-byte struct. It's currently stored as the special xattr com.apple.FinderInfo, and still houses some things that are used today, such as the alias-bit, the item's color (primary tag in Mavericks), and the creator-ID and file-type (if the file has those).
and search for finder_flags.
If you want to convince the modern Finder to open the ancient Finder, you'll have to delete the com.apple.FinderInfo xattr. In theory, simply clearing the system-bit should suffice, but I'd have to look up the details on it. A Terminal command that removes the xattr is:
xattr -d com.apple.FinderInfo path/to/your/ancient/Finder
Opening ancient-Finder won't show anything, because it has no data-fork, only a resource-fork. All the executable code is stored as CODE resources.
There's more to FinderInfo, but to get all the dusty details I'd have to dig out my old paper copies of "Inside Macintosh".
After some experimenting, it looks like the Finder-flags have no effect. It's the file-type and creator-ID that elicit the alert from modern Finder.
Using the SetFile and GetFileInfo command-line tools:
I turned off each of the attribute bits in turn. The ancient Finder file still wouldn't open.
Then I changed the creator-ID to "????". It still wouldn't open.
Finally, I changed the file-type to "????". The icon instantly reverted to a "generic document" icon, and I could open the file.
Then I changed the creator-ID back to "MACS", and I couldn't open the file again.
FWIW, setting the file-type to "FNDR" on any file will cause it to appear with the classic-Mac Finder icon, and the modern Finder will also refuse to open it.
Also, more docs on the FinderInfo struct, the bits in the flags, etc.:
The struct member is fdFlags. The bits are defined in the nameless enum whose first item is kIsOnDesk.