I've been trying to puzzle out some of what's going on with the numbers.
Well, my advice is don't - Tesla may change things up on us at any time.
But, I believe the current best thinking (I know I'm asking for it here) is that:
a) Signature models come first. That's 1000 cars all equipped exactly the same way with all options, modulo color.
b) Roadster Owners, Friends, & Family next. That's the next batch, currently in the 400's.
c) General Production next.
Typically US has been first, but since the idea behind the Signature was to have the first 1000 cars all the same for production efficiency, I wonder how they will handle things like RHD and such for UK/Australia. They may do all LHDs first then then RHDs. Again, no-one outside of Tesla seems to really know.
Also, for General Production, there's been rumors that cars specced with the 300 mile battery will be produced ahead of cars with the 160 or 230 mile battery, even if the sequence number says otherwise. Again, that was to give the production line as much uniformity as possible. There was a lot of discussion around that here and other forums, but again, Tesla is officially silent.
Note that these groups are apparently additive. That is, R1 should be car #1001, but P1 will be something like #1451. It could be more: if, for instance, 100 more Roadster owners put deposits down before the Sig run is complete, then P1 could be #1551, for instance. And again, we don't really know. And we don't even know if they're doing all US first, or some combo or sequence of countries.
Similarly, current Signature model reservations total less than 500 out of a possible 1000. Does that mean if they don't reach 1000 by the time they're into producing Signatures that they'll stop early and go to Roadster owner production, or will they do a complete 1000 series no matter what, and then have "spec" cars ready to sell off the lot while people with reservations for custom orders have to wait? Again, no-one who knows for sure is talking.
You can drive yourself crazy with this. My advice is: don't. Either you'll get an early car and that's very cool, or you'll get your car later and the reward for waiting will be a more sorted out vehicle with less problems and maybe even some improvements - although it's been reported that Tesla feels a lot more confident in the early Model S's than they were with the early Roadsters. Time will tell.
Oh, and just because you have an early reservation number doesn't mean you have to take early production. My reservation form said that there's a "skip me now" option if I'm not ready to spec and order my car when they're ready for me. I don't know how far back that bumps you in the line, though (all the way back, or just as far as you want, or something in-between).