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.