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