Well, this is an interesting example for reverse engineering. In order to achieve hardness in excess of 64-66 HRC, you have to do something different than conventional atmosphere carburizing (gas carburizing) or low-pressure carburizing (vacuum carburizing). Adding ammonia to the gas mixture during atmosphere carburizing is called carbonitriding, and can lead to increased hardness after quenching and tempering. Personally, I have never seen 68 HRC for this process, or even reported in technical literature, more like 65-66 HRC as a maximum. There are some treatments like ion nitriding that can create very high surface hardness, but usually the depth is quite small, not > 0.5 mm to 800 HV. If you are really interested in understanding what is going on with the sample, you will need some further testing that is quite specialized like elemental mapping of the near surface layer (SEM w/ EDS, maybe Auger analysis) and X-ray Diffraction.