Hi Theoderich,
Both the jck and the knight's helm are marks of Josef Carl Klinkosch, a major Viennese firm. I'm sure if you search the forum you will find more information about the company.
Regards, Tom
Diether Halama states in his monographic work »Die Gold-, Silber- und Metallwaren-Fabrik J.C. Klinkosch in Wien (1797-1972)« (ISBN 3-9500741-0-4), in there published mark's register (p. 150-152) that the polygonal mark was J.C. Klinkosch Export mark (p. 152, Nr. 20). The knight's helmet in profil, to the left with open visor mark (in an vertical oval frame), D. Halama register to of course (p. 151, Nr. 12).