What is the name of this type of mark?

Is there a name for this mark? It sits below a letter as a means of showing an abbreviation. (In this case, Established abbreviated to Estd.)

I've seen it used in the form below in place of an apostrophe for a contraction. Frequently, it's used to abbreviate William to Wm.

I know there's a diacritic mark called an underdot, but I'm not sure this qualifies as a diacritic since it's not meant to influence sound value of the words.

It's some sort of stylized period (full stop for my British friends). But, I'm wondering if there's a term for using it to raise the letter above it in this usage.

Solution 1:

It's a stylised underline. From Wikipedia...

The practice of underlined (or doubly underlined) superscripted abbreviations was common in 19th-century writing.