I'll help clear this mud up a bit.

C and E steppings refer to the Penryn product with 6MB of cache.

M and R steppings refer to the Wolfdale product with 3MB of cache.

The changes from C --> E and M -->R are equivalent and are pretty much the same changes just on the 2 different products.

Revision changes are done most of the time to iron out bugs and make manufacturability changes that lead to higher yields and better binning flexibility.

The stepping change was done to add in some features that have been developed but weren't quite ready at release time of the original design.