Milosh - a Special Projects number was assigned when that aircraft was designated for any non Standard AAF maintenance or special treatment from others of its class. Could be assigned by the customer such as UASAAF Material Command representative at NAA plants for unique equipment like Very Long Range Radio sets or at a Field Depot such as BAD2 Warton to designate the intended use of 130/150 Octane Fuel for P-51 in the ETO (and BAD1 Burtonwood for P-38 and P-47).
The Secial Projects Log contained all the Special projects numbers which in turn listed all the Technical Orders which applied (i.e. engine maintenance, plug changes, etc for 150 octane fuel)
Often wrong but rarely uncertain..