A custom profile lathe tool was ground to cut the lug for an external bayonet adapter for the D-7.
Originally a hacksaw blade (shown) was used but was subject to breakage. A 3/8" HS steel cutting blank was ground and slotted with a cut off wheel to produce the profile in one operation.
The tool is upside down and the cut made on the far side of the lathe for visibility. 2" PVC Sch 40 electrical conduit has the perfect ID size. ABS or PVC DWV pipe was found to be unsatisfactory because most is "cellular core" and does not work well for turning. Unfinished blanks with this lug are available. They are very handy for adapters of many kinds such as close up lenses and filters.