I think that Steve is right to say that Marx's argument
does not prove that members of an equivalence class must
share a common element.
However, leaving Marx's argument aside, one can say that
the nature of the metric space defined by the equivalance
class of commodities is such as to be characteristic of
a system governed by a scalar conservation law, which is
in effect what Marx goes on to assert about labour content.
I have a paper produced jointly with Allin that is about
to be circulated by AF to those of you on the value
working group that argues this point in more detail.