Gunsmith or machinist?
Modern "gunsmiths" are machinists, pure and simple. In the far off days of the 1940's, an apprentice was given a task like this and a set of files. If he couldn't accomplish the task, he was automatically called a ribbon clerk.
Any gunsmith who CANNOT accomplish this, is certainly not of the old school.
It was done this way: give the part a coating of layout blue (also called toolmaker's ink) and carefully lay out your lines with a very sharp scriber. Then you apply a straight edge guide for the file (one was deemed enough in those ancient days.) This was usually a hardened piece of tool steel. Note that it was a major operation to harden the guide without warping it.
Then the guide was clamped to the part SOLIDLY with toolmaker's clamps. The part (grip frame, in this case) would be solidly attached to the workbench or vise. You begin with short strokes, laying the file against the guide. Soon, the cut will be deep enough to guide the file evenly against the guide piece.
The first cut would be narrower than the slide rail, in this case, and the depth kept shallow. (The file has to be guided by the left hand as the right hand pushes it, making an even, parallel cut.) Then the other side is done. After this, the fitting phase begins in the normal fashion.
Before this kind of test, the "old master" called you "Mr. X" If you accomplished your task successfully, you were Mr. X, the Gunsmith." No higher compliment could be paid to an apprentice who had proven himself a worthy journeyman.
Today, any kid could do the job with minimum instruction. Clamp the part, align the cutter and go!
And, for this, I say: How many of you Misters could do this job as your long ago predecessors did it?
Is it "Progress" that we now depend on electronic readouts and special lead screws? Whatcha gonna do when the lights go out? (and, they will go out for the big time this Summer.) Think about it.
Boy, we have a bunch of (by their own admission) ribbon clerks hanging around here, don't we?