custom-filter-face-spec is a compiled Lisp function in `cus-edit.el'.

(custom-filter-face-spec SPEC FILTER-INDEX &optional DEFAULT-FILTER)

Return a canonicalized version of SPEC.
FILTER-INDEX is the index in the entry for each attribute in
`custom-face-attributes' at which the appropriate filter function can be
found, and DEFAULT-FILTER is the filter to apply for attributes that
don't specify one.