The biggest change in Marple chisels other than owners, place of manufacture, and of course quality as the years passed was going from hammer forged to drop forged. As best I can tell drop forged chisels begin showing up in the '30s and by the '50s most of the chisels were drop forged with maybe the exception of firmer chisels.
I have some of both hammer and drop forged. Other than the hammer forged having square necks vs. drop forged round necks the only other visual clue is the ferrel. The hammer forged ferrel is defiantly brass on the drop forged chisels I'm not sure because it has a strange color and stays shinny with little or no aging.
A photo of the two styles. The top and bottom chisels are hammer forged with square necks, the middle two are later chisels with round necks.
At this time most of my chisels are the earlier hammer forged (three drop forged) and as I can find replacement chisels for the round neck ones I will hopefully end up with all early chisels.
What strange folks we tool collectors can be. The quality of both in use is about the same (maybe all my round neck ones are from the early years of drop forging) but I want all square necked chisels or at least a complete set. Then in use I will probably use both interchangeably.