There's nothing to worry about: the "MSC" refers to the National Benefits Center, which originally started out as as the Missouri Service Center (MSC). Since the application is initially processed at the NBC before being passed on, the receipt number starts off with MSC.
From the National Benefits Center FAQ at
http://uscis.gov/graphics/fieldoffices/nbc/faq.htm
What role does the NBC play?
To serve USCIS Field Offices, the NBC will complete all necessary pre-interview processing of I-485 Adjustment of Status cases, including conducting background security checks, initial evidence review, adjudication of associated I-765 and I-131 applications (filed with the I-485 or separately), denial of adjustment of status cases for applicants who are statutorily ineligible, and forwarding scheduled cases to the USCIS Field Office for adjudication.
A unique function of the NBC is that the Center utilizes a Lockbox facility, located in Chicago Illinois. The Lockbox is a bank under contract with the Department of Treasury that performs fee receipting, fee deposit and initial data entry for specified Forms according to business rules established by USCIS. The Lockbox then forwards all properly filed cases to the NBC for the pre-interview processing steps listed above.
What is the National Benefits Center (NBC)?
The National Benefits Center was originally created as the Missouri Service Center (MSC), which was set up to process applications associated with the LIFE Act. On April 16, 2003, USCIS Director Aguirre approved the transition of the Missouri Service Center (MSC) into the National Benefits Center (NBC), which greatly expanded the scope of the Center’s operations. The NBC was primarily established to serve as the hub and conduit for all USCIS field offices by completing pre-interview processing of Forms generally requiring an interview. However, evidence of our origin remains, as all Forms receipted by the NBC still bear the receipt number prefix of “MSC.