Looking at my parts catalogues it could be a later starter fitted from 1953 on which doesn’t have the arm?
Googled for the Solenoid bit on top via the part number 701838 or 701857 but no joy – I’ll add a parts image once I re install my scanner software 🙄
Maybe someone will know better, but you may have to fit a new starter motor
EDIT: is this the one Baz?