Sometimes abbreviations are made by placing the terminal characters in superscript and underlining them, as in "1st," "Mc," and "No" (see examples below). What is this process called, and what is a general name for any one of these abbreviations (the abbreviation a whole, not the superscript characters alone) so made?
This seems to be known as an Ordinal Indicator.
Here's the blurb for convenience, some formatting lost from Wikipedia like the actual subscripts:
In written languages, an ordinal indicator is a character, or group of characters, following a numeral denoting that it is an ordinal number, rather than a cardinal number.
In English orthography, this corresponds to the suffixes -st, -nd, -rd, -th in written ordinals (represented either on the line 1st, 2nd, 3rd, 4th or as superscript, st, 2nd, 3rd, 4th).
Also commonly encountered are the superscript (and often underlined) ordinal indicators º and ª, originally from Romance, but via the cultural influence of Italian by the 18th century widely used in the wider cultural sphere of Western Europe, as in 1º primo and 1ª prima "first, chief; prime quality".
The practice of underlined (or doubly underlined) superscripted abbreviations was common in 19th-century writing (not limited to ordinal indicators in particular, and also extant in the Numero sign №), and was also found in handwritten English until at least the late 19th century (e.g. "first" abbreviated 1st or 1st).
I'd be surprised if there were a name for this type of abbreviation. In British English, abbreviations have a full-stop after them. E.g. "Esq." for "Esquire". Contractions are less standardised. Modern practice in British English is usually to simply omit the full-stop. E.g. "Mr" for "Mister" (this distinction is rarely made in US English). Where the contraction causes a change in pronunciation, an apostrophe is used. E.g. "shan't" for "shall not". In the past, up until the late C19th, contractions were often shown by placing terminal letters in superscript and underlining usually followed by a full-stop - this was particularly the case in variants of secretary hand. This was done with a wide variety of words that we would not longer consider writing in this way - such as Mr., but also names (like "Wm. " for "William"). The practice persists in exceptional cases - notably with ordinals (e.g. "1st") and some Latin abbreviations (e.g. "no. " for "numero").