Hi Sophie
I am not sure that there is a simple answer to your question - it may depened upon the design.
For example, in the case of the Saint Louis honeycomb design weights (which were not made every year) editions for 1990, 1997, 1999 and 2006 have the acid etched numbers. The ones made in 1973, 1974, 1988, 1989 and 1991 are only numbered on the certificates. So if there was a move to label all the weights it was after 1991.
Alan