I will leave others to comment on the field blouse and shirt (although I don't have much hope for them), but there is one thing I can tell you for sure:
The trousers are a post-war repro made by Mil-Tec (Sturm). The alleged Reichsbetriebsnummer ("R.B.Nr.") is their phone number. (Which I think is a clever little joke.)
The number as it appears in my repro field cap (this time, with the "0049" country code for Germany):
Andreas is a genius.