Based on ChiMan's comment, I think I might have got it: On the cases that will take a V72, the uppermost ridge will bisect the holes for the crown and pushers. Cases that won't will have a top ridge that falls short of reaching the equator.
Unfortunately, it turns out that my beloved FAP 6239 has one of the latter cases, a discovery made by The Zigmeister during a recent service. I don't know how my old watchsmith managed to cram everything inside and get it running, but Zig refused to touch it in that precarious state. The guts have been transplanted into a new DW case, which jmb will soon be working on.
Ignorance was bliss!