Notation in Frege's Grundgesetze der Arithmetik: The U with a flourish
The symbol stands for the currency ``Mark.'' It is an old symbol developed in handwritten manuscripts. As far as I know it is a lowercase m with an abbreviation symbol to indicate that letters are dropped. The lowercase m has changed to a simple horizontal bar.
See the OLD FLOURISH MARK SIGN on page 146 of http://folk.uib.no/hnooh/mufi/specs/MUFI-Alphabetic-2-0.pdf
Frege used an unusual German Fraktur font for the fancy U. This has created many problems for modern typesetters, as one can read in a 1982 edition: "After unrecallable arrangements had been made for composing the book, it proved that Gothic letters (Frege's deutsche Buchstaben) were not available."
The sharp angles and ligatures in the fancy U are characteristic for a Fraktur font, but there are many variations. I have searched the web for precisely this U, and have not found it.
These typographic issues are of course quite unrelated to mathematics, but not entirely; see "Maths = typography?"
http://www.tug.org/TUGboat/tb24-2/tb77lawrence.pdf
Unrelated to the original question, but noteworthy in this context, is the question how to typeset Frege's symbols in a modern document. Fortunately, this is possible with Metafont and LaTeX (the fancy U is \fgeU), see
http://soliton.vm.bytemark.co.uk/pub/jjg/en/code/fge.html
Frege uses Fraktur ("Gothic letters") for quantified variables, lower case for first-order variables, upper case for second-order variables. The "fancy U" does not belong to these.
The "fancy U" is similar in style to ligatures of standard abbreviations, like old signs for weights and other measures. The sign for "Pre" is pretty close (214C):
http://www.unicode.org/charts/PDF/U2100.pdf
If the crucial 'P'-bit wasn't missing, this could almost be it.
The custom characters Frege uses look much more ham-fisted than his elegant character, compare his character for "Endlos", his sign for the smallest infinite cardinality (right side of the equation):
http://homepages.uconn.edu/~mar08022/pics/1-122-150.pdf
(The character on the left that looks like a script 'N' or 'A' is an overturned 'lb'-ligature, for "libra": pound.)
This speaks for the "Fancy U"'s being a symbol that was present in the type-setters box.