Hari
The DeVallière 140 feedbox allows to pick up certain threads *ONLY*
Those threads are 0.25mm, 0.30mm, 0.40mm, 0.50mm, 0.60mm, 0.75mm, 1.00mm, 1.20mm, 1.50mm, 2.00mm, 3.00mm et 6.00mmn (multiples of the leadscrew pitch of course)
For those threads and those threads only, you can stop the carriage and reverse it to start the next pass using the feedbox lever or the stop dogs. You will pick up the thread again without any problem, with the spindle running in the same direction all along the operation.
The 140 feedbox has a particular feature wich allows a fast reverse to starting point : when threading from tailstock to the chuck, the feedbox will work with the selected pitch. But on the way back to the starting point for the nedt pass, the feedbox will drive the leadscrew directly, whatever pitch you selected. That's why it is important to perform the threading operation only with the lever set on the right.
For any other thread, you must NOT disengage the nut from the leadscrew or you will loose the thread. So you have to stop the motor and reverse it to get the carriage back to the starting point.
I am not aware of the existence of a thread chasing dial for the DeVallière 140, although one could certainly be made from scratch without problem. The 6mm pitch of the leadscrew is pretty common.
Hope this helps.