Mea culpa. Forgive me. I gave up too easily. I finally found it in "A Genealogist's Guide to Old Latin Terms & Abbreviations" at https://blog.genealogybank.com/a-genealogists-guide-to-old-l...
Instant (often abbreviated “inst.”): This term refers to a recent occurrence in the present or current month.
Ultimo (often abbreviated “ult.”): Ultimo or Ultimo Mense is a Latin term/phrase that refers to an occurrence from the previous month.
Proximo (often abbreviated “prox.”): Proximo refers to something that will occur next month.