Thank you all, who answered my question.
Eventually, I asked people at Alexander Schleicher egelflugzeugbau GmbH, and they sent me this reply:
"Dear Mr. Cwikla,
The device you asked us about, just connects the lines from the static ports in the fuselage (You have to do this somewhere).
It averages both pressures and distributes the resulting pressure to
four lines out.
Since you only need two, the other two lines out are closed by interconnecting them. There is no magic about it, it is just hollow.
Part No is: 150.11.0007
Best regards,
Michael Greiner"
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____________________________________________________________
Alexander Schleicher Segelflugzeugbau GmbH
D-36163 Poppenhausen; Alexander-Schleicher-Str. 1
D-36161 Poppenhausen; Postfach 60
Frank,
C-GZBY,
15448