- Found it
In addition to the 12 hide types for Level 1, Level 2, and Level 3, one more, final available type has been hidden and listed, if not yet found:
Letterbox: Pharr Mill Historic Site - OU0AFC
This makes 13 types hidden. The 14th type - challenge - does not seem to be available for me to hide; however, my "unknown" hide for Level 3 was a challenge hide, for what that's worth.
Thanks, OCNAChallenges, for setting this up!