While passing by KSSI this morning I observed a Beech 350 running up in front of one of the hangars. It was white overall with green & gold accent stripes, similar to what's on FS9's Beech Baron. Nothing remarkable about this aircraft until it began its taxi toward the EOR. As it passed my vantage point I noted it had a single six-digit number - 161511 - on the ventral strake, and absolutely no other identifying numbers or insignia. It's the only ostensibly private aircraft I've ever seen that did not have an "N" number. Numbers were about 8 inches high, in black paint.