Warbird Information Exchange

Hei,

Is there anyone who can help me to find information about the gunsight C 3 that was used by the army air corps arround 1938? Was this gunsight also exported to other countries? I ask this for a friend who is very busy with the history of fokker and he found letters were the Estonian government was asking if the Fokker D 23 could be equipped with this gunsight. Now he like to know more about this gunsight.

Thanks in advance.

Best regards,

Mathieu