Where should the serial # for Series 1 be? I trying to check out a bass...

Depends on the age.  Very old instruments have it struck into the top of the headstock.  Old, but not that old, have it struck into the fingerboard below the 24th fret.  More recent instruments (definitely by 1990) have it engraved into the truss rod cover.
 
Not sure what they do when you go continuous wood plates - maybe on the back side of the truss rod cover?  There should also be at least one number somewhere inside the control cavity.

Ok, I don't find one in any of those places; I'm thinking of posting photos, to see if there anyone who recognizes it.

Apparently, with the instruments that had the serial number on the fretboard; when an owner would take one to a local shop to have the fretboard replaned, the serial number would be sanded away.

I think I found it on the headstock.