I am not really a uniform and photo guy so I my experience in dating photos is non existent 🙂
I have just read the Teno chapter in the Angolia book which is very detailed , including the uniforms but the helmets section is lacking the examples I posted and which are apparent in most photos even those in the book. Helmet collectors are at this point not even 100% sure on the Teno helmet decals (no decals in photos I have seen) and I have not had the opportunity to study one.
In the Angolia book up to Uniform 1936 they do not speak of helmets. Nor does he speak of them at a later stage other than that they would have worn black civil helmets with decals which is strange because all period photos in the book show apple green medium duty edelstahl helmets or Heer transitional with 2 decals.
Estimated guess on my part would be 1938/39 - as the Medium duty helmets we see with Heer decals are Always double decal. Could be 1938 also. Why do I think this ? Because Teno moved across the border with the Heer to Austria (Anschluss) and in 1939 into Poland. It would make sense that they'd use helmets.
Double decals were dropped for the Wehrmacht in 1940.
It is a very complicated subject to say the least but it would be great if we would find more photos. I have a post card incoming with a Teno soldier with medium duty stahlhelm. Will post it when it arrivés.