Short summary: This was a standard way to represent a double sharp at the time of printing, but the representation was changed afterwards and will not be recognized easily by musicians today.

The fact, that some features were surprisingly stable in musical notation over the centuries receives little attention compared to those features, which changed significantly, see similar questions here concerning the representation of rests and clefs.

A Liszt-contemporaneous German reference, Meyers Konversationslexikon 1885-92, has this article for Doppelkreuz (double sharp):

.

For completeness here the link to the article.

So no, this is no misuse of a different symbol as suggested in a comment, but one example how double-sharps looked like in that time.

Short summary: This was a standard way to represent a double sharp at the time of printing, but the representation was changed afterwards and will not be recognized easily by musicians today.

The fact, that some features were surprisingly stable in musical notation over the centuries receives little attention compared to those features, which changed significantly, see similar questions here concerning the representation of rests and clefs.

A Liszt-contemporaneous German reference, Meyers Konversationslexikon 1885-92, has this article for Doppelkreuz (double sharp):

.

For completeness here the link to the article.

So no, this is no misuse of a different symbol as suggested in a comment, but one example how double-sharps looked like in that time. Interesting side-note: the article mentions two even older symbols for the double-sharp but claims that the sign in the example from the question is among those one would normally use.

The fact, that some features were surprisingly stable in musical notation over the centuries receives little attention compared to those features, which changed significantly, see similar questions here concerning the representation of rests and clefs.

A Liszt-contemporaneous German reference, Meyers Konversationslexikon 1885-92, has this article for Doppelkreuz (double sharp):

.

For completeness here the link to the article.

So no, this is no misuse of a different symbol as suggested in a comment, but one example how double-sharps looked like in that time.

Short summary: This was a standard way to represent a double sharp at the time of printing, but the representation was changed afterwards and will not be recognized easily by musicians today.

The fact, that some features were surprisingly stable in musical notation over the centuries receives little attention compared to those features, which changed significantly, see similar questions here concerning the representation of rests and clefs.

A Liszt-contemporaneous German reference, Meyers Konversationslexikon 1885-92, has this article for Doppelkreuz (double sharp):

.

For completeness here the link to the article.

So no, this is no misuse of a different symbol as suggested in a comment, but one example how double-sharps looked like in that time.