The first use of ecthed markings like these was in 1977, presumably introduced for the anniversary weights, to give an air of formality.
Having said that, some have this etching (like Crown Bubble and Jubilee Moonflower), but others are still fully hand engraved (like several Jubilee floating crown examples I've seen).
It's not clear what the policy was (if any) but 1977 Lobster has also been seen etched with the three-row etching:
<name of weight>
Etching was then introduced to all unlimited weights in 1978, and some limited weights.
It seems to be random whether Limited weights were etched or hand engraved- but it may be Perth/Wick variances