Branded as "Boley Leinen" this rare item was intended for lapping work and the polishing of screw heads in watches and pivots--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 inserted into a collet and the iron disc charged with a lapping compound such as oilstone dust and oil. The left hand was used to rotated the spindle backwards and forwards (grip being aided by the knurled surface) with two fingers of the right hand being used to rotate the lap (slightly faster than the left) and, at the same time, press it against the part to be treated. When the full surface had been polished it was cleaned and iron lap was replaced by the bronze unit charged with diamantine. The final cut was given by the boxwood lap and 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 a small screw with its head set to protrude through a hole in the end of the unit so that it could be polished (the part to be polished was secured by the 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.