After some more digging around online, it was suggested that I install Belarc Advisor, which I did. This program locates "missing" Microsoft (and other) software updates. It found many missing monthly security updates, going back to last December, along with MS links to the direct downloads. They all installed fine, up to and including the September 2017 security update. However, the September Monthly Rollup installs still says they have failed (one recommended and one optional). I am thinking that this is at this point somewhat erroneous, and we shall see if the October update installs, when it's available.