In the paper Magnetic Bloch analysis and Bochner Laplacians of Ruedi Seiler, this expression appears in this context:
The conventional notation for a fiber bundle would be $\pi: b\ \to\ M$. where $\pi$ is a continuous surjection from $b$ to $M$. Does $\varnothing$ mean a continuous surjection? Is this a conventional notation for this? Or does it mean something else here?
Looks like bad OCR to me. Below screenshot of the paper from different source.