The family and I took a trip up to the Manassas Battlefield Sunday afternoon before the Super Bowl. Here are some updates to National Park Service frequencies:
172.7250 [NAC 47C] Manassas NBP
170.0500 [NAC 5C2] Monocacy NB (this frequency may be shared with Antietam NB)
170.3625 [NAC 4C5] this frequency was weak from Manassas, but has the characteristic Park Service NAC. I couldn't make out who it was, but suspect it may be C&O Canal NHP. A user on RadioReference has also reported hearing 169.6875 [NAC 4C5]; this may be another C&O repeater.