Problem resolved.
The issue was due to an alias (C-Name) associated with the Blade Enclousure's OA (1 of 2). My theory is that when SIM polls for further information after feeding it the OA credentials, it discover the "Enclosure Name" and performs both forward and reverse name lookups, and due to the mismatch it decides to append the Serial to the Enclosure Name, as previous stated - HPENC0001_YY3990ABCD
Example:
Enclosure Name: HPENC0001
Serial #: YY3990ABCD
Keep in mind the OA has a different name and DNS entries, e.g. HPENC0001-OA01, and HPENC0001 does not exist in the DNS (as you can't reach it via the network).
Once we removed the Alias, the Blade Enclosure was named correctly in SIM.
Hope this will help others in the future.
J