Now, that's an answer! Thank you very much!

Actually, I had Eric Rasmussen's site in my bookmarks for a long time. I just didn't know it had all those detailed information. So, according to the part number (690-5064A), it's the original System 3.0/Finder 5.1 that shipped with early Pluses. It seems to be labeled "Macintosh Plus: System Disk" instead of "macintosh Plus: System Tools", though.

I guess I'll ask the seller if he ever tested the floppy and knows he's selling an incomplete bundle…

Thanks again, slomacuser!