An old brass wide angle rectilinear fitted to a #4 Ilex Although there are some drawbacks outlined later, here is the technique known as "front mounting a lens" In this method the complete barrel lens with its own iris is adapted to fit where the front element of a conventionally installed lens would be:

First, a piece of suitable aluminum stock is clamped in the chuck, faced and trued.
The outside is turned to size and threaded to fit the shutter.
Before boring out the inside, while the metal stock is still thick and strong a knurling tool is applied to give a finger grip to the adapter. This is a useful convention which cues the user that it can be gripped and unscrewed easily. (The metal would be crushed by the tool if this operation were done after the inside is bored -- thin metal is knurled best before it is thin.)
Switching to a boring bar all the inside dimensions of the adapter are made including the base size for the internal thread as well, as in this case counterboring to get the lens barrel's shoulder as far inside the shutter as practical.
The last step, finishing the inside is to cut the thread to receive the lens.
After all accessible work is done, all threads tried and matched, chamfering, polishing, etc. the piece is cut off from the stock with the parting tool. Every effort must be made in planning the job to do as much as possible (external threads, internal threads, etc) in one setup. This ensures that the inside and outside will be perfectly concentric.
The adapter as is looks just cut off from the base stock. (the previous pic is blurred because the lathe was turning during the exposure.)
To finish the front of the adapter after it is cut-off it is fitted to a specially made collet (in this case to duplicate a #4 Ilex shutter) This ensures a true run and no risk of damage to the threads.) I have an inventory of special collets in all popular shutter and filter sizes.
After counterboring to (in this case a very thin wall) allow the barrel lens to fit back into the adapter and near the iris the fit is verified in the lathe.
The finished adpater is very light and thin. It is designed to have the lens as far back in the shutter as possible for reasons I explain next

This sketch shows an ordinary setup for a lens. Front element in front of the shutter and back element screwed into the back of the shutter.    The part labeled "shutter" could just as easily be a "barrel" and many lenses are made so as to have an easy interchangeability if the barrel has the same dimensions as an available shutter. Note that the iris of the shutter is used to obtain the f-stops.    The complication and drawback of the "front mount" system is clear in this slightly exaggerated sketch. Note that when the cells of the lens are left in their original barrel and the entire assembly is fitted to the front of a shutter a much larger shutter is needed to accomodate the lens /iris barrel assembly.    Imagine a cone of light rays emitting from the back element to the film; it is necessary that a large enough opening be provided so as not to cut off (vignette) the image. In the conventional setup shown in the first sketch this is never a problem. The "front mount" method also has the disadvantage of limiting shifts and tilts available to the user.    This is not to say that this method is not entirely practical and economical if the user knows the limitations. The end result is often just as good (as in the case shown in the photographs) as any. Be aware, though that this method often shows up as a large, cumbersome, piled up adapter kind of disappointment which was not envisioned.