I've had several of those derailleurs and I've never seen the ferrule you referred to. I always assumed that was a cable stop for bikes that routed the cable through a short section of housing at the bottom bracket. I could certainly be mistaken about this...

Edit: cool bike by the way! I built a World Sport from that era for my wife, and she got plenty of use out of it. I'd recommend upgrading the brakes, they were pretty bad on my wife's bike.