"DLT3C" is not a STEPPING code, it is an OPN code. The OPN code identifies the key specifications of the chip in order: Packaging, Voltage, Die Temperature, Cache Size, and Front Side Bus.
The STEPPING code is the next line on the cpu label and is 5 letters ending in either an "A" or a "B" for the Thoroughbred cores. The "A" or "B" in this stepping code identifies the Thoroughbred revision. You want to buy only "B" cores since the "A" versions are not very good overclockers (I only get an extra 300MHz out of my 1800+ T-bred "A" which also has an OPN containing "DLT3C").