28 - week (i.e. week28)
B - AFAIK there is a A and B series
121 - NFI

I think I read in xtremesystems,

28 - week (i.e. week28)
B1 - Batch Production Code
21 - NFI

It's a code to differentiate between the batch of production. ES, OEM and Retail. Not so sure though. I'm still on AMD.