This is my understanding of what "forging" means in this context.
The earliest process just used a hammer and an anvil. At the Tools for Working Wood site there are some nice little videos from the Ashley Iles factory which show this. I can't work out how to do an easy link for these, but go to this page
https://www.toolsforworkingwood.com/sto ... 12.php?v=v choose the third video, "A visit to Ashley Iles pt 1, Forging" and advance to 31 seconds in. You can see how the bolster is formed by hammer work at the edge of the anvil. Later in the same video, at about 2' 34" in, there's the same process, but with a mechanical hammer (a "goff" hammer).
The process of drop forging needs less time, skill and energy, so when it came in, it soon displaced traditional forging (except for small scale specialists such as the Iles family.) In drop forging, a hot billet of metal is smashed between a pair of dies to make the shape, in one or two passes, then the surplus metal is trimmed away. This is clearly shown in a "How It's Made" video at the Buck Bros plant in the USA - this link should go to one minute in as the hot billet emerges to be placed between the dies -
https://youtu.be/8fAIdYZYU20?t=60.
When did this change happen? I have a catalogue from an exhibition arranged by Ken Hawley in 1992. One of the exhibits listed was a Stanley 5001. The catalogue says that this was the first Sheffield chisel made by drop forging rather than hammer forging, in 1963.
According to Roger Ball, Marples started offering their "Splitproof" range of plastic handled chisels in 1954. We know that later examples would have been made by drop forging, but if the statement above, that Stanley were the first to use drop forging in 1963 is correct, it's possible that the early Splitproofs were forged in the old fashioned way.
Indeed, it seems that Marples seized on this difference and tried to capitalise on it as an advantage, by using the somewhat meaningless phrase "fully forged" to suggest that the newfangled drop forging produced an inferior product. Roger Ball notes that some examples were marked HAND FORGED SHEFFIELD ENGLAND on the top of the blade and also shows a leaflet from 1961 saying "all chisels and gouges are hand forged. This method does not disrupt the molecules of the steel as does production drop forging."
However, he also shows that later Marples chisels have round necks, therefore they must have been drop forged, like the Buck chisels in the YT video.
Presumably Marples invested in drop forging not much later than Stanley, in the early sixties.