Note that this info is out of the box only.
You system might have been upgrade therefore the Part Number is not a valid way to actually decipher your Release (Rls) number but does tell you what it started out as in case needing to replace the hard drive, then upgrading back to the release it was.
To decipher what release number you actually have on a working system you must log into Element Manager and view the System/Identification/System Software Version.
Where your version number starts with 1, 2, 3, 6, 9 or 10
Example where version starts with 9:
Then the release number is Rls 5 on both BCM50 and 450
BCM50 SRG the release number is already shown under System Software Version
Version 1.00.2.04g, 1.00.2.04 or SRG50 R1 is Rls 1 NT9T6500 Year 2006
Version 2.00.2.05c, 2.00.2.05e or SRG50 R2 is Rls 2 NT9T6501E5 Year 2007
Version 6.0.2.05 or SRG50 R3 is Rls 3 NT9T6502E5 Year 2008
Version 126.96.36.199.572 is Rls 5 NT9T6505E5 Year 2009
Version 10.0.1 or 10.0.2.52.258 is Rls 6 NT9T6506E5 Year 2010
Version 8.0.1.00.301 is Rls 1 NTC03100SYE5 Year 2008
Version 188.8.131.52.594 is Rls 5 NTC03100SXE6 Year 2009
Version 10.0.1.02.120 is Rls 6 NTC03100SWE6 Year 2010