Rolex Submariner with NO case reference number markings

[Whoop!]

I have been given a gold Rolex Oyster Perpetual Date Submariner. Serial # 922****. I detached the bracelet from the watch body and found the serial number at the 6 o'clock position but the 12 o'clock position is blank. No case reference number there. Nothing there. The information I have been able to find indicates there should be a case reference number in the 12 o'clock position.

A jeweler has inspected the watch and said there is information inside indicating it was last inspected/serviced in 2000. He didn't know why there was no CRN on the exterior.

Does anyone know why the 12 o'clock position is blank??

Thanks.