[RolandF]

The sign above the Sütterlin "u" is a curved horizontal penstroke to distinguish the Sütterlin "u" from the Sütterlin "n". "Umlaut" "ä" "ö" or "ü" are indicated by two small vertical strokes, originally signifying a small Sütterlin "e". The two dots of nowadays German "Umlaute" are remnants of these two vertical strokes.
So these Sütterlin "Umlaute" are in fact two letters: "ae", "oe" and "ue".
(Something out of the history bag)

Regards

Roland