If you have 55 pages of his records from the National Archives you do not need the Soldiers and Sailors System Number, Just sent them a copy of a couple of the most important pages.
However, K C found the Soldiers and Sailors listing also.
The Soldiers and Sailors System index number mearly confirms to the Veterans Adminstration that the National Parks Service at lest found a record for the person. That relieves some of the pressure on the applicant (you) to provide all those records.