Edit: thanks to a tip from @grasses4
, who informed me that the IOOF (which I read as 100F) stands for the International Order of Oddfellows, I was able to find that this is an Oddfellows symbol meaning “cheerful giving” and benevolence to those in need. You can see more about their symbolism at hmb-odd.org (I'm sure other places as well, that was just the website I opened, and it seemed very detailed).
Is anyone familiar with this symbol? I've tried doing a reverse image search, and I looked on findagrave to see if there were details about it. Didn't see anything, so I thought I'd ask here.
White Oak Cemetery, Forreston, IL, USA.