With regards to the production code the four letters may have the coding as with the Thoroughbred B cpuz. In that case the third letter would be something to do with the production batch an "A" meaning the first production batch(reputed to be the best for overclocking). An M in the third position would be made from a mixture of left over wafers from the production. Processors with marks A to C are meant to be made from the purest wafers and should overclock the best.
So something like **AW would be from the first batch. **BW from the second batch and so on. **MW would be a cpu from a mixed batch of wafers.
Stedman