I downloaded the PDF from the Library of Congress website. I don't have access to Cataloger's Desktop. I suspect the lines have something to do with revisions but I could be wrong. I'm looking at instruction 6 if that helps.
From: Program for Cooperative Cataloging <[log in to unmask]> on behalf of Prochazka,David <[log in to unmask]>
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Subject: Re: [PCCLIST] Lines in SHM
David Procházka | Music/Special Materials Cataloger | The University of Akron | Bierce Library 261C | Akron, Ohio 44325-1712 | 330-972-6260 | [log in to unmask] | https://works.bepress.com/david-prochazka/
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What do the vertical lines in the margins beside text in the Subject Headings Manuals mean?
The specific instruction I'm looking at is about adding a parenthetical qualifier after an abbreviation in a 4XX field (H373).
Osgoode Hall Law School Library