Hi all,
I apologize for the late response.
The part I've posted (pb1130411) has, actually, an external reference and it causes some problem with the "mirror operation".
But my question is oriented, not on to mirror this particular piece, but in general.
What is always present in the mirrored parts made with "File > Save As > Mirror Part " is the feature named MIRRORED MERGE (in the Steven picture it has the name: Mirrored Merge id 6287).
What's the problem: the problem is that I can't add features before that particular mirror-merge feature in the model tree, and this is a limitation for a future modification of the piece.
For example: if between 3 weeks I'll have the necessity to make some changes at the "base piece", naturally I want this changes also on the mirrored one, but I have the limitation that I am not free to modify the mirrored piece as the "base piece". If the part is simple, I get along; but if it isn't like this I'll be in trouble.
(This happens also using Model > Create in the assembly and selecting sub-type Mirror)
So I look for the way to make a mirror part without this limitation but I got the idea that is impossible with ProE and you have to do by hand all the times...