BenQ-Siemens P51 is a PDA phone produced by BenQ-Siemens.
HMS Unseen (P51) was a Royal Navy U-class submarine built by Vickers-Armstrong at Barrow-in-Furness.
LÉ Róisín (P51) is the lead ship of her class of offshore patrol vessel in the Irish Naval Service.
The North American Aviation P-51 Mustang is an American long-range, single-seat fighter and fighter-bomber used during World War II and the Korean War, among other conflicts.
The P-38, developed in 1942, is a small can opener that was issued in the canned field rations of the United States Armed Forces from World War II to the 1980s.
The Parker 51, introduced in 1941, is a fountain pen.