Based on an initial assignment of 46 properties to "generation 1" custodian sales (there are a few more to be decided, so more will probably be added), I've written an XSLT transformation that discovers and lists details of all subsequent titles which are from the "next" generation, in that they include properties which were transferred as part of the previous generation sales.

There are some complications: some titles include a mix of custodian and non-custodian properties. Still waiting to hear from JS-R and SF about how they would like to handle this; also, there's a strong possibility that even "pure" titles may have lost properties that were on the preceding title.