Milling a fixed sight frame for adjustable sights isn't really that hard. I have done a couple N frames and a couple K frames. All the frames except J frames are the same except for length of front piece and screw hole location.
Remove cylinder and leave out yoke screw. Mount in mill vice right behind using brass pads without lower screws in side plate. You want them out so that side goes flat against pads. Level top of frame. Center up and use a .250 end mill to cut slot. Mounting piece is .276. so you need to move over a bit each way to clean it up and get mount blade to just slide in. Make the cut for the adjustment assy. Then do a regular slot cut for where the elevation nut rides. Then do the under cuts for the elevation nut base. A cutter for this is hard to find, but, I made a couple out of small 2 flute end mills by using a dremel to remove material on the end mill so only 2 small "teeth" were left on tip of mill. Some on has siince found a cutter that will work and posted a link to it in the gunsmith section. Any tool for this will be a bit fragile but your only removing a very small amount of material, using slow light cuts. Now mount sight with elevation near all the way up, with gun still in mill. Set up a drill directly over the mounting screw hole. slide out sight and drill hole. Then without moving the guns location mount the tap in the mill and turn it by hand. This is the best way to tap small holes as you keep perfect alignment of tap on hole. This is huge on very small taps. If the barrel is still mounted some sight mounting holes come out over the barrel shank and you have to be careful with depth of drill and then start hole with starting tap and then switch to a bottom tap. Easier if barrel is not mounted and your probably going to be installing a adjustable sight barrel anyway.
This is an example. What started out as a 1917 with a couple issues.
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