For what it's worth, I am copying some "distinguishing features" of Probus coins from the various mints from RIC Vol. V-2, but to be honest I cannot see many of them on the coins which are definitely attributed to this or that mint (e.g. because the legend combination and image and mintmarks were not struck by any other mint) here on wildwinds.
However, the information about the letters M and V is useful, because distinguishing between seemingly identical coins from Antioch and Siscia can be especially difficult.

Antioch

• The lettering is regular and generally good. The busts are well-designed and proportioned.
• The length of the neck differs from engraver to engraver.
• The letter M: The strokes of the M are often disjointed.
• The letter V is joined at the base, sometimes as a usual V but often by a short base line.
• The bust can be shorter and squatter than that of Siscia.

Cyzicus

• The letter V: RIC mentions that "the lower part of the letter V slopes to the right of the viewer" but on the coins definitely from Cyzicus, on wildwinds, I cannot see anything special about the letter V.

Siscia

• The letter M: The diagonal strokes generally begin at the tops of the vertical strokes and the strokes are joined together (unlike Antioch, see above).
• The letter G: (especially on aureii:) The lower part of the curve is long and becomes narrow towards the end and often, before the end it supports a wedge or anvil-shaped block distinguishing it from a C. This was not used at any other mint, however, Siscia also used other forms of G.
• Later issues show Probus' face as grave, dignified and quite long.

Serdica

• The letter M is often engraved much resembling an N. Where it is engraved as an M the diagonal strokes begin low down on the vertical strokes.
• The letter V have strokes slightly less perpendicular than e.g. Siscia. but are almost always unjoined at the base.
• Coins of Serdica were struck on flans larger than those of other mints.
• The portraits are generally bold, grave and large and the bust longer from the top of the head to the base line.

Ticinum

• The letter M: The diagonal strokes generally, but not always, begin beneath the tops of the vertical strokes and seem to be on and in front of them, nearer the viewer.
• Mintmarks which begin with a Latin officina letter P,S,T,Q and end with a T (e.g. PXXT) are from Ticinum.
• Coins of the AEQVITI series are from Ticinum.

Tripolis

• The letter M: The diagonal strokes almost always begin low down on the vertical strokes.
• The letters of the legend often vary in size.
• The busts occupy a greater portion of the field than other mints and the lettering is less regular.
• The emperor is always shown as radiate and draped right, with the cuirass, if any, indicated beneath the drapery on the breast.