[Mr70]

That's right DaveC.
Your 71 Monte Carlo LS-5/TH400 A/C equipped used that (4 core) P/S tank,along with the 3 core D/S tank denoted as I A,and the overall prefix tag BR.That was the Heaviest Duty radiator available for that particular vehicle.
That radiator was very popular among 1969-71 Chevrolet A & G bodies,part #3019205.