I think Pfaff 335-17 is a binding machine subclass (so the machine was originally a binding machine). Quote from the instruction book for the 335 old casting:
'The operational instructions given in this booklet pertain not only to the ordinary Pfaff 335-17 binding machine, but also to all other subclass varieties. The individual subclasses are fitted to suit different operational requirements. The various subclass organizations differ only in the type of rigid or swing-away binder or tape guide mounted on the feed-synchronized attachment bracket. An exception are the Pfaff 335-9, -09, and -09-33 which are specially designed for portfolio work and have a rigid tape guide mounted on the cylinder arm. The Pfaff 335-332 X 251 occupies a special position in that it is fitted with upper feed acceleration control rather than ordinary unison feed '