information about 69138 and 79138

[marincathy]

Would appreciate answers to some questions about Oyster Perpetual Lady Datejust models 69138 and 79138

I would like to decode reference # R6913883GB8570 (circe 1989-1990)

I understand that 79138 followed 69138 sometime around year 2000.

What year did 79138 start using solid gold links and when did they start engraving serial # on the rehaut?

The answer to these questions would be greatly appreciated by me, a fairly new member of TRF and Pledge membership.
Thanking you in advance, Catherine (aka marincathy)