Harmonica Tab

The harmonica tab was basically a 1-to-1 mapping of the notes to the corresponding hole, and thus, is a type of numbered musical notation. For each note, it will indicate the number of the hole to play, direction of breathing (in or out), and even either bending (usually for diatonic) or "slide-in" (usually for chromatic)
One methodology for indicating direction of breath is by showing the direction of arrow; another is by using either a "+" or "-" sign, or "i" (for inhale) and "e" (for exhale). Bending was shown with a bent arrow with the direction of breath, or by a circle that circle the note, or even a simple line next to the breath indicator. Additional lines and/or circle may be used to indicate how much to bend.
For example, on a key "C" diatonic:

To indicate button press on Chromatic, a similar indication to first level bending may be used.

The breath indicator may be placed right next to the hole number, or below the number. Same for bending/button press indicators.

To indicate the beat, on arrow system they may use the length of the arrow. However, the more popular method would be to use a slightly simplified notations, such as "o" for whole note, // for half notes, "/" for quarter notes, "." for eighth notes, and place them above the characters, while spacing them accordingly.

For chord, they will simply show the numbers to play, so for example:

However, they may simplify it, especially when playing blues. For chords, it was common to just play three or two holes instead (sometimes even just one), especially when the instrument is not of the same key. For example, in blues progression in G (G G G G7 C C G G D7 D7 G G) it's common to use C diatonic, and use the following: