I was born in 1978 and was "adopted from birth" (as in left the hospital with my adoptive parents and never with my biological mother). The legal process of the adoption was completed about 9 months later.
To cut a long story short, I have recently become more interested in my adoption, especially with the "myth" about it that came from my grandmother.
Anyway, I have always had a copy of my adoption certificate but just recently received a copy of my original birth certificate. On the certificate it is filled in as you would expect, signed by the registrar and directly under box 17 it has written "Adopted, Registrars name, Registrar" - it is the same handwriting and name as signed above and is certainly not in any margin.
I have no idea when it was added but the practice doesnt seem to be that historic as it happened on mine in the late 70s