Hi Stephen
Do you mean a Boxford CSB? If so then the necessary info is available in the Files section of the Boxford Lathe Yahoo group (you will need to join the group and yes, I know the user interface is very poor). However, you say there is no screw-cutting on the lathe. Does it have a leadscrew? If not then what you have is a training lathe and adding screw-cutting means finding, at minimum, a leadscrew with bearings, quadrant, an apron, tumbler set and changewheels.
This website will help with identification http://www.lathes.co.uk/boxford/
Richard