What Do Western Digital Model Numbers Mean Whats The Difference Between These Two Hard Drives According To Their Model Numbers Being Different?

Whats the difference between these two hard drives according to their model numbers being different? - what do western digital model numbers mean

I have an RMA number for my Western Digital Raptor 150GB 10K RPM, I am not the model number WD1500ADFD-00RAR1

WD1500AHFD back 00RAR5

Of course, the only difference is the final figure was just curious, what exactly does the last digit of the difference between the two.

Thank you!