it is correct, that the rifling changed from six to four lands. This was done…. hum, well, I don't have the necessary files at hand presently. As far as I remember, this was done around 264xxx or 265xxx, maybe a little later.
What do you mean with "zinc alloy"? Are you referring to the frames? Anyway, the early ones were completely made of steel. No zinc alloy at all. Also later examples were made of steel – except for the trigger, what really was made of zinc alloy. Change occurred in the…. well, I guess, 325xxx or so SN block. From 370xxx (?) on, also the floor plates of the magazines were made of zinc alloy.
Yes, the grips of my 260214 has a dull appearance, what is correct on early grips. The material is the same as on later grips. Compare with yours: inside is a symbol consisting of a "M" and a "D". This logo is a standard for German plastics. Inside this logo you'll find a number "43", what is the code number of the maker (Dynamit Nobel Abt. Kunststoffe, Troisdorf). You also will find the letter "S". This is the code for the material composition. The early dull looking grips also have the material composition "S" (later grips were marked with composition codes T1 or K). The dull appearance simply results from fine "pits" on the surface, which aren't present on later, shiny looking grips.
BTW: the red color on the rear sight on yours is original. The early examples did have front sights painted yellow and rear sights painted red. Around SN 270xxx the sights weren’t painted anymore.