@mwl Hmmm....
webdns=> select serial from zone;
serial
------------
26906
25975
252888627
2007102006
28697
26886
22213
184
45238
253322835
(10 rows)
Ok, that fits. The one zone that actually used the data-based scheme (probably because it was imported from a manually maintained zone file) is an empty in-addr.arpa zone that hasn't changed in 17 years.