It looks to me like an argument in favour of allowing <idno> inside <msItem>.
Alternatively. I am tempting to say "use @corresp", but fear that would bring Wrath upon my head.
Alternatively again, the next TEI release adds @source to some elements (via an att.source attribute class) for
a similar purpose (see http://bits.nsms.ox.ac.uk:8080/jenkins/job/TEIP5/lastSuccessfulBuild/artifact/release/doc/tei-p5-doc/en/html/ref-att.source.html
for pre-release). maybe <msItem> should be included in the list (currently quote, q, writing)? or is that concept different?
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