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 # 22.214.171.124 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.