## Trouble with old notation and an unknown function space

I'm reading an old paper and I'm trying to figure out the name of one of the function sets being used. As best I can tell the set appears to be $\{f| \int_{\mathbb R} t |f(t)^2|'dt <\infty\}$. The set has an associated symbol but I don't recognize the font and/or the letter/symbol being used. Any help is appreciated.