Flying through that narrow corridor without radio of GPS navigation is a pretty daring concept. Using the combination of a twelve year old terminal area chart and the FS 2004 map (for the special rules boundary) there doesn't seem to be any prominent waterways or highways running the length of the corridor. A couple of modest size reservoirs might be useable to mark the north edge of the NW entrance to the corridor. Suburban (W18) and a closed strip (not shown as such in FS, but near or at intersection INKOE) might be useable to mark the boundaries near the center. I don't see anything that looks prominent near the SE end.They couldn't have been very observant to get within ten miles of the government center though. The Capital Beltway (a WIDE limited access highway) keeps pretty close to a ten-mile radius around the government center in the Maryland suburbs of Washington. FS 2004 is showing the special rules area as a fifteen-mile radius with a few flat sections.
Robert
Near KORF