Smaller cover franked Washington 3c Red (1860's) cancelled with the 'bullet' WASHINGTON D.C  DEC 4 date stamp (no year). Addressed to a Mr John Townsend Lakeville MA (American CW Research Database shows 2 soldiers named Townsend from Lakeville, MA. Both served in the 29th MA infantry).

To reverse neat cachet cancellation FORWARDED BY THE SANITARY COMMISSION.

( The Sanitary Fairs helped hospitalised soldiers write letters home. I would suspect your cover is one of those). 

Condition as shown:

ID: US 45516