Not all the time do they receive different part numbers. Intel for example, issues different part numbers on processors that they deem "perform better" and many times are the same processors used for entry level processors (identified by batch number, such as what has been seen in the Intel I7 series). The "unlocked" processors are able to withstand the abuse of being unlocked, while hacking a locked processor to obtain overclocking capabilities with the same batches of that of the unlocked processor has yielded negative results - usually a burned out processor.