I found this from Migration FAQ of Classic Metalink available on
How do I view the old Service Request number on my Service Request?
As part of Oracle's move to a single system for tracking SRs, SR numbers were changed.
You can view the Classic MetaLink SR number in the Legacy SR Number field in the SR views on My Oracle Support. To add the Legacy SR Number field to the Service Request list region click the table icon in the upper left hand corner of the list view. Select Columns > Legacy SR. You may sort on this field by clicking the column header.