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2. V. Vujta, Feingerätetechnik, No. 3 (1957).
3. L. Meyding, Techn. Zbl. Prakt. Metallbearb., 59, No. 4 (1965).
4. W. Lotmar, Techn. Rundschau,59, No. 9 (1967).
5. G. B. Kainer, Author's Certificate No. 159,999, Byull. Izobr., No. 2 (1964).