No necessarily so.
Ive had a mail from Detlev who's pretty good at this Rolex stuff
and he tells me that there are regular
retailed 18K pieces that carry the serial inside.
This is because the watches in question were marked with Import marks (UK, Portugal etc) and therefore the case backs were removed in order to do this. It seems that if Rolex ever remove a caseback for an addition prior to delivery they add the serial inside.
Obviously a civilian 1665 DR Seadweller carries its serial inside and was sold retail however this seems to be the only exception. That said, the 1665 was a special watch at that time and one that was destined for professional use.