We should listen carefully to what Roy Fielding said.
A careful reading of what a "Location:" header should mean in
this context is exactly what we need. It identifies [an URI,
today an URL] by which the body may be refreshed. Under these
circumstances, the "Content-" prefix only serves to increase the
count of "RFC 822 headers of record" and should be eliminated.
In fact, even if a Content-ID is known for the part this could be
handled as a kind of URN and referred to using the Location:
header. Its natural interpretation would yield the right sense.
Al Gilman