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?"
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