Seeking file containing a letter RE: Removal of charge of desertion (Civil War) - Record group 94.1

In researching the Civil War military service of Simeon Felix Sears, I found this item:

General correspondence with U.S.Military Pension Office 

The indexed correspondence is not in his pension application file, which makes no mention of desertion and only notes that Michigan can find no record of him. I note this letter was dated 25 Oct 1890 vs his pension application which was filed 4 Oct 1890.  How could I obtain the correspondence related to his request for removal of the charge of desertion?

I obtained his pension application file.

His pension application includes a card 3-216, which under "other claims" lists No. 470915.

Included in the pension file materials is his His Soldier's Application 20 Sep 1890 in which he claims he had applied for a pension about 10 years prior.

I would like to find more information about his service, or lack thereof.