serial number order

[yoda888]

I did a search but couldn't find my answers here or on the interwebs. I suspect it's because the words I was using are far too common.

If I have 2 watches with say Z serial numbers. The first one is z609001 and one is z990001, is it safe to assume the z99 watch is "newer" and built later in z-series?

i assume it is, but wanted to check that it was in order and not random. Eg, Z999 could be the first watch and z000001 could be the last watch if it's random.

Also can there be duplicate SNs for different watches? So say an Exp2 has z123456, would an Exp1 also have z123456? Or is the SN exclusive and not used again in other rolex models?