Thank you! If it's in part of the receipt number above the letter of approval then it's MSC, National Benefits Center.
Hope that helps. I have honestly lost track of the whole process and had my 3yr mark for eligibility for citizenship marked which was actually now. I was content with my 48 month extension even though it's a little more of a hassle to travel with and makes checking in early almost impossible.
Wishing you all luck!