Not P-51Bs from the brits.
The first two ex-RAF "Mustang I" planes have been shipped in December, 16, 1941, last aircrafts have arrived in the USSR on May, 14, 1942. One of first planes in June - July 1942 was tested in Scientific research institute of Soviet Air Force. Most of technical parametres were not pleasant to the Soviet experts; they liked only powerful weapons.
Most part of those ten "Mustangs" was transferred in 6th Reserve Brigade (the Commander - colonel Shumov), where P-51s were used as trainers. For example, in Summer, 1942, during the training of personnel of 1st ferry-flights Division (these men should know features of American aircrafts), the five P-51s were used. Three of them were used long enough, and were based on aerodrome in Ivanovo town.
Three "Mustangs" from August, 22, 1942 have been directed in 3rd Air Army (Kalininskiy Front) for front trials. Two of them were used in 5th Guard Fighter Regiment, which was equipped with Lavochkin - Gorbunov - Gudkov LaGG-3 fighters. Mainly, on these P-51s the Regiment's Commander V. A. Zaytsev flied; but other pilots also tested these aircrafts - but planes were not pleasant... No one real air combat flight was made. Soon, the airscrews of those two fighters were damaged, and they were returned back (because of lack of spare airscrews).
One of ex-RAF "Mustangs" has been received by Training Regiment of Zhukovskiy-named Air Force Academy (Moscow); in 1946 it was still in the using there. Later this plane served as an element of exhibition of aviation technics. Another "Mustang" was in exhibition of Bureau of new technics in Central Aero-Hydrodynamic Institute (Moscow).
Thus - no any mentions about "the Northern Fleet" in history of Soviet "Mustangs".
The Northern Fleet was the user of another American fighter - P-47.
Russian P51's