Despatch documents often (usually? almost always?) contain a number in their <head> tag, which represents their incrementing number in the year in which they were written. This is distinct from the CO registration number, which was only given to despatches and other documents received and registered in the CO. This number has been missing, up to now, from the <teiHeader>, because it was in a range of different formats which made it slightly more difficult to parse out when the original conversion was done.
Today I put a few hours into this, and I think I've solved it. In the process, the following new items have been added to the metadata in the header:
<idno type="despatchNo">1</idno>
<idno type="despatchType">Executive</idno>
<idno type="documentType">Secret</idno>
The first should only appear in despatches, as should the second; the third is a parallel to the second, in that many of its values ("Financial", "Judicial", etc.) are shared, but it applies to documents which are not despatches. This is the theory, at any rate; proofing will show how consistent this has turned out to be.
One consequence of this is that we'll be able to update the search page, to add a new element for "despatch number"; and also, I hope, we'll be able to make dates a little more precise, so you can search for a proper date range. I'm planning to work on this next week.