To commemorate an International Youth Festival (Moscow 1957), KMZ issued some cameras with special Festival engravings. At least 5 Festival cameras existing: Zorki-C, Zorki-2C, Zorki-3C (very rare), Zorki-4 (rare) and Zenit-C (uncommon). Those cameras were apparently not otherwise different from mass-market variants.
My Zorki-C Festival camera has the serial number # 57150339.
Known serial numbers:
# 57136602 ___ Juhani
# 57138879 ___ A.Nikitin
# 57142579 ___ A.Nikitin
# 57150339 ___ SovietCams
# 57143190 ___ Omega