In the factories that I have worked in the designer would draw any changes that he required. The drawing would be sent to the prototype dept. ( probably consisting of 2 or 3 skilled fitter/ machinists). and complete article would be made. It would be most unusual to modify an existing product. The finished prototype would then do the rounds e.g. testing, costing dept.etc.so would have to look presentable. In my view the pistol shown would not be good enough.
Mac