It seems to me that, the first letter being a high letter in the alphabet that it marks a cpu to be used on the lower speed cpu's. 'C' seems to be what you would want, 'L' seems to be for the lower speed core too. A 2600+ Sempron I bought was a 'L' stepping.
At least, that seems to be what AMD did with the Socket 'A' cpu's. 'AIUGB', and 'AIUHB' steppings was better than 'JIUCB' steppings. You never saw a higher clocked Socket A cpu's with a 'J' stepping, only the 'A'. The 1700+ and such speed cpu's used 'J' steppings for the cores.
Of course different processes and such will change the stepping letter. So, the letters should be based on the cpu's using the same process.
I think that the first letter being high is a indication of it failing at the high end sector and that marks them to be used in the lower speed cpu's.
But of course, I don't have any 'Concrete' evidence to back up my claim. Just the experience I have had. So in other words, you can take it with a grain of salt

Jason