• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

6C33C tube stamp


I purchased some 6C33C tubes on Ebay and found that some of them were labeled with the stamp below.


I decoded the text as "rechecked" or "retested" but my question is about the "X". Is this a tube that failed quality control?