Thank you for contacting the Intel Communities.
Intel adds the markings of the processors with a very difficult to erase method.
Different scenarios may be present for the mentioned CPU's.
1. They were erased with some sort of abrasion tool or surface, this may affect the heat spreader of the processor (metal plate were the markings are) The purpose of this procedure may be overclocking purposes.
2. This may be an engineering sample (it may say "Intel Confidential" only), please keep in mind that these type of processors are not for sale, only for developing purposes they are meant to be used.
Do not hesitate to advise me if you encounter with any additional questions or concerns with this.