[...]note the rates of descent are much lower in the old Brit document, due to the much lower disc loading (and power available) of those early helicopters.[...]
This is not the Reason.
The reason is to be found in the shape of the early blades, of whom are designed to distribute the same lift(by massflow) on each diameter.
As you can see in the document the blades thicken up to the inside as seen from above.
Modern blades wont do that, so the inner part of the blade has an lower lift resulting from the lower speed, this does not support the outer vortex. Ergo: higher rate of descend possible without getting into VR.
Greetings,