This set appears to be made for the early narrow sprockets. Does anyone own this set and can confirm it will fit an early Panzer III/IV narrow sprocket?

Hmmm,
The early/narrow track is listed as 36 cm and the later/wider types are listed as 40 cm but those 40 cm appears to be measured outside of the track bolts so the actual track link is only 38 cm wide. On the other hand, if the same measuring fault is committed on the 36 cm track the result is 38 cm.
The later/wider track came in two or three different types and the first of these is also sometimes referred to as "early"

On the other hand, these are definitely the early narrow track used on the first types of Pz III / IV and StuG's:

/ Robin

Edit: See below. Final answer

Here is a picture of the track on an early Dragon Pz.Kpfw.III Ausf.G (6639). I did not add the resin track pins but the fit is excellent.

It's a great set. Yes, it is suitable for Pz-III Ausf D,E,F and StuG III A. I installed these tracks on my Pz III F