@mikeypru - Yes, exactly - version 6.3 L70, designated for another scanner, was installed in my iMac but used to operate the ScanSnap S1500M. I had to drill through the Fujitsu support website and select the other scanner (S1300i) to get to the desired software download page. As you indicated, the “current” version of ScanSnap Manager for the S1500M is version 3.2 L91.
Why install the “incorrect” software version? Long story - the short version is that this was a workaround recommended by Fujitsu about a year ago to deal with an incompatibility between ScanSnap Manager and macOS High Sierra v10.13. Apple modified the portion of macOS dealing with PDF files and Fujitsu was slow to respond with a software update. Here is a link to Fujitsu’s explanation and instructions:
https://talk.macpowerusers.com/t/scansnap-s1500m-will-be-doa-on-mojave/7676/22
Perhaps the version 3.2 L91 ScanSnap software might work correctly for the ScanSnap S1500M with High Sierra and Mojave now - I don’t know. You can check by scanning a multi-page (at least 5 pages) document in the ScanSnap scanner and saving the document as a single PDF file. If the pages are in the correct order, you should be OK with this software - at least until another macOS update that “breaks” the ScanSnap software. Again.