The retroflex letters are among the places where a Gurmukhi hand shows its training. Ḍadda, Ṭainka and their dental counterparts share a common construction, a stem from the headline with a bowl or a bar attached, and the differences between them come down to which side the attachment sits on and how far it closes. In a careful hand the four are unmistakable. In a rapid one they converge, and the reading of a manuscript can turn on how a particular scribe habitually finished a bowl. That is one reason Gurmukhi palaeography works the way it does: rather than looking for rare letters, a reader characterises a hand by its treatment of the commonest constructions, and the retroflex and dental families give the largest sample.
Printing changed the problem rather than removing it. Metal type made every instance of a letter identical, which resolved the ambiguity of the hand, but early Gurmukhi founts had to decide how far to regularise shapes that manuscript practice had allowed to vary, and the letters of these two series were among those most visibly standardised in the process.
There is a further modern chapter to this. Digital Gurmukhi has had to settle the same questions again for screens, where the letters are rendered at sizes far smaller than either a manuscript or a printed book usually used, and where the distinguishing features of the retroflex and dental families are exactly what disappears first. Fonts designed for the script rather than adapted to it treat those features as the ones to protect.