Were all the Argentino 1909s made in Berlin?
No they were not.
.. I am assuming the 1909 is a good receiver to use for a custom build. Am I right?
Yes. good and preferred by many, but they still require a substantial financial outlay to bring
the complete action up to spec. if a person is planning a top flight/high standard build.
If you used say another great action like a Brazilian 1908.. gave it a full grind of all surfaces
to true the whole thing up, have the inside rails and surfaces hand stoned/polished, etc, etc,
and put all the best modern upgrade goodies on it, you wouldn't know whether it was originally
a 1909 or 1908 or some other military Std.98.
The manufacturing standard of 1909s and 1908s mostly just means there is less to contend with
if doing a full high level Mauser action build.
ITs the same reasoning that top guys like D'Arcy Echols will carefully select which Pre 64 M70
to use for a build, cause some are terribly rough from the factory.