Solution : https://service.sap.com/sap/support/notes/854523 (SAP Service marketplace login required)
Key words : 
manufacturer part number reason, logistics - general --> interchangeability, release ecc dimp 5, expected default state, sap reference img, mpn conversion exit, active conversion exit, conversion exit, customizing transaction, parts --> set
Related Notes : 
       
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| 987610 | Long Material number and MPN are delivered as default | 
| 961410 | Add. info. on upgrading to SAP ECC 6.0 SR2 ABAP | 
| 913971 | Add. info. on upgrading to SAP ERP Central Component 6.0 SR1 | 
| 826092 | Add. info. on upgrading to SAP ERP Central Component 6.0 |