MY LORD IT'S HOT (Summer attire questions)

[Flat Foot Floey]

Indeed. German sportswear is rare. But why do you think it is 30s and not 50s? Not that I say it isn't but detective work is interesting.
Did you ever see a (german) catalog pic?

 

[Fastuni]

Indeed. German sportswear is rare. But why do you think it is 30s and not 50s? Not that I say it isn't but detective work is interesting.
Did you ever see a (german) catalog pic?

I'd also love to see photos or illustrations showing such shirts!

You once posted a German magazine from 1950 - there were some variations of "exotic" summer shirts that are going in a similar direction (but much more gaudy and already quite "Americanised"):

This one is from 1949... text mentions that loose shirts, worn like jackets, will increasingly replace safari-jackets and light sportcoats.
They all look rather "fancy" and some appear to be influenced by US fashion:

As my shirt is unlabelled and probably tailormade, the date is just an estimation on my part.
Of course there is a good chance that it is 40's or 50's.
But I see nothing that would speak against 30's.
It was actually offered as a "WW2 tropical uniform", but it clearly is civilian.

The collar with "shawl lapels" at least is certainly a kind of spearpointy "Schiller collar" variety, similar to this 1928 image you posted:

The overall quality and treatment of pockets, interlining etc. is quite similar to my German 30's summer sportcoat: