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It *is* a Carl Zeiss Distagon f/4 50 mm destined to be sold in the former East Block countries. The only difference between these Optons and the version available in the "West" is the name printed on it, and the serial number.
The ownership of the name "(Carl) Zeiss) was disputed after the break up of the firm when the second world war ended and parts of it were moved to the "West" to become Carl Zeiss Oberkochen, and parts of it remained in what then became the GDR.
The name "OPTON" is supposedly taken from *OP*tische Werke *O*berkoche*N*. Though i'm not sure that it really is.
The same dispute also entailed the ownership of registered names like Distagon. That's why the version made for export to the East Block were marked "DI" instead.