Is the one at the beginning of Zeiss lens 51 a high imitation?

No. A query on the Zeiss official website shows that the company will print the lens number at the beginning of the lens model to prevent counterfeit products. The number starting with 51 is the Zeiss lens number, so the Zeiss lens starting with 51 is not a high imitation. The Zeiss Group develops, produces and sells measurement technology, microscopes, medical technology, ophthalmic lenses, photography and cinema lenses, binoculars and semiconductor manufacturing equipment.