In reply to James Mouw:

Your question piqued my curiosity and I went browsing through my ANSI
standards for serial holdings statements at the summary level.  Under
rule # 4.5.1.3 provision is made for a second level designator if
necessary to prevent ambiguity by a colon.  Table I gives an example
of 12:6 (volume 12, issue 6).  I find this publication invaluable.
Hope this helps your dilemma.