I'm working on reclassifying some of the microfilm from arbitrarily made-up call numbers to Library of Congress call numbers. Apparently when we first started to collect these titles they just numbered them sequentially as they arrived. Microfilm F1, F2, F3, and so on. Nothing is grouped by subject, at least not on purpose. That's why they're getting reclassed.
It makes for some pretty funny shelf-reading. Here's what I ran across today:
Microfilm F1760-F1760b -- National Underwriter (Property and Casualty Insurance Edition), and its two title changes (numbered F1760a and F1760b, so they'd sit together).
Microfilm F1761 -- Skiing.