@Mandi Barrett If you run into more abbreviations you're not sure about, you could check out this FamilySearch Wiki page: https://www.familysearch.org/wiki/en/Abbreviations_Lists_for_Personal_Names_(English)
I have used the last link on the page a lot. It is about traditional nicknames. It was very interesting to go through the list of nicknames. For example, my given name is Patricia and my nickname is Patty which is also a nickname for Martha.
@Mandi Barrett It usually is an abbreviation for William.
@Mandi Barrett You're welcome!
Wm = William
Jno = John
Rch = Richard
Ths = Thomas
Jos = Joseph