Buzz:
It is called ghosting. I have re-cased receivers both ways. To reduce ghosting, pinning and using old screws and pins during your blocking as you fixture and pack the crucible.
All double guns new and re-cased are cased with the hinge pin in place and if you look carefully at all the pictures posted here and above or anywhere, original and re-cased, there
is no ghosting around the hinge pins. This pretty much answers your original question.
Most of us doing case colors try to achieve as near the original "factory" case colors as possible. Many factories "did not" pin or screw the holes as it was time consuming and sometimes pin removal was a pain after heat treating. After all, they were in business to make money. But even with the same manufacturer, many times variations occur, in other words sometimes they appear to have been pinned and sometimes not. CCH is like a snow flake, no two are the same.
A well done re-case
by a competent tradesman or shop on a complete restoration, where the entire gun starts out fresh again, is near impossible to distinguish from an original factory color case hardening procedure.
But there are so many "gaudy" over the top re-case jobs out there with all the wrong parts cased that were actually blued, that it is usually quite easy to spot a re-case. Like the gaudy one pictured above from a well known shop where the forend iron is cased and should
not be as it is an "extractor" gun and L.C. only cased forend irons on "ejector" guns. Then there are the ones with bone that should be cyanide and vise verse. It is very much like many of the Field Grade and other grade guns out there that have been re-stocked with XXX wood or greater which really look out of place, it's the same with case colors, too much and it becomes a circus piece. To me these "out of place" upgrades look like white wall tires on a Ferrari.
With the proper amount of homework on exactly what a specific gun looked like as it left the factory, most 'good' case color tradesman can duplicate very closely the original look desired.
Some examples of guns that I have done with ghosting for the factory original look and
no ghosting for the same original factory look where it applies after proper home work and research:
1892 Marlin no ghosting.....
Top L.C. Smith 20 no ghosting.....