My original NLM export code (from TEI to NLM 3.0) was written based on the old system of <biblStruct>s used in the bibliographies of the first two volumes. I'm now using a much simpler <bibl>-based system inherited from the Mariage project, so I had to add more code to handle this. I now have the NLM 3.0 export generating good valid bibliography lists. The NLM 2.3 is generated from the 3.0, so there's no need to change anything there.