Geo.WKB.encode-exclamation-mark

You're seeing just the function encode-exclamation-mark, go back to Geo.WKB module for more information.
Link to this function

encode!(geom, endian \\ :xdr)

View Source

Specs

encode!(Geo.geometry(), Geo.endian()) :: binary() | no_return()

Takes a Geometry and returns a WKB string. The endian decides what the byte order will be.