My understanding, is that the gents that made those, we're not at all regular or consistent with the numbering system used, or very faithful in getting them out the doors entirely in any particular sequence, making it a bit of a requirement to have a solid handle upon both the serial numbers, and the various combined features which were added or subtracted, during the production life of the various versions, in order to reasonably firmly fix a manufacturing date.
YMMV, but that is what I have read, mostly here, in conversations about this subject.
Cheers
Trev