I'm not sure if Spector has different "series" of numbers based on USA-built vs Euro-built vs Korea/China/Indonesia-built, if they simply run consecutively, or some combination of the two. Actually, I realizing seeing some "behind-the-headstock" shots that they have some type of differing system for USA Basses, having seen SNs for "rare-ish" basses such as NS-2Js (USA Bolt-Ons with Ash bodies and 2 EMG Jazz pickups - basically USA-made [Invalid or Expired Link Removed] reissues but with the standard Spector bridge and Crown Inlays) but I believe that for the Mass Produced Euro & Far East Neck-Thru & Bolt-Ons they're straight by build date and are therefore hard to track a D/O/B based on them.
I highly doubt there's any type of "secrecy" going on - simply between the early days (late '70s/early '80s) when they weren't production line basses, to when Kramer bought the line they may have instituted their own numbering system (including their own USA and Import lines), when Stuart restarting building under the "SSD" name in the early '90s it was again a low-number shop (approx 12 per month) until they started making the Czech Republic-built NS4/5-CRs and Korean-built lower-budget basses in the mid-90's, and the regaining the "Spector" name as a Trademark which significantly increased their desire and output as well as being able to rename the SSD-era basses back to their "proper" Spector names... I simply think there's been a great deal of change over Stuart's and the company's career, far too much to come up with a more "logical" serial numbering system. I also echo mobis.fr in dropping PJ an e-mail: he typically gets back to you within a day or so, and can be extremely helpful in tracking down information or answering questions.