It is an interesting practice within the watch industry, to return or not to return the parts being replaced during service or repair. Thus far, my experience with the various brands that I have personally sent or the experience my friends had, it is not a common practice to keep the replaced parts.
Parts returned to the customers. I am sure you guys can figure what brand the parts are from. :)
Yet another. Not the same brand as above.
To me, I understand why some brands do not return the serviced and replaced parts (prevent old parts from appearing in questionable places) and I think that it is a good practice. However it would be nice if the service centre do show you (at the time of collection) the parts replaced. Sadly this is not a common practice.