Those roller guides were typically used on Mixte or Ladies frames to route the brake cable so that it could reach standard brake bridge mounted brakes. The brake cable would leave the levers, go down the "top-tube" then up the seat tube, make a U-turn around the pulley and go down to the cable hanger of center pull brakes. It was mounted on the seat post bolt and positioned underneath it.
Had one on my Raleigh lady's bike. It worked well.
Amir Avitzur
Ramat-Gan, Israel