Branded as "Boley Leinen" this device was intended for lapping work, the polishing of screw heads in watches and pivots and reducing the length of tapped shanks - and may also have been employed in snailing work on such items as flat, keyless wheels. Normally this type of device incorporated some sort of driving mechanism - usually mounted in a lathe toolpost and driven by an overhead - but this example is intended for use by hand when clamped in a vice. Supplied with the unit was a collet holder and a set of horological collets, the smallest of which was able to hold a 24 B.A. screw. Either two or three disc-shaped laps were provided: the two-disc set had them in iron and bell metal (bronze) and the three-disc set an additional one in boxwood. In use the screw or other item to be polished was held in a collet and the iron disc charged with a lapping compound such as oilstone dust and oil. The left hand was used to rotate the spindle backwards and forwards by either a bow, with the string running around one or other of the grooves seen at each end of the hexagonal bar, or by rotating the bar itself with the palm of the hand. Two fingers of the right hand were used to turn the lap (slightly faster than the left hand) and, at the same time, press it against the part to be treated. When the full surface had been polished it was cleaned and the iron lap replaced by the bronze unit charged with diamantine. The final cut, given by the boxwood lap, resulted in a deep, blue lustre.
Three miniature "lanterns" were included in the kit - shown top right in the photographs and bottom right in the boxed set in the catalogue illustrations. These had finely machined stems and an adjuster screw passing through into the head space wherein could be inserted the small screw to be polished, with its head set to protrude through a hole in the end of the unit (the part to be polished was held in place by a long screw passing through the body of the chuck). Should any reader have experience of using this device, the writer would be very interested to hear from you.