The 351 4V HO for '71 IS the Boss 351 engine. And other than a few very uncommon one-off special orders or dealer engine swaps, these engines were only installed in Boss 351 Mustangs's for 1971. (I think I said the same thing twice... LOL)
Each and every 1.806 Boss 351's will have their VIN as 1F02RXXXXXX.
So...
In 1972 the R codes were what many refer to as the "detuned Boss 351 motor". And forgive me if you already know this, but these (1972 motors were available in ALL 1972 Mustang body types. (unfortunately I do not have the details for Grande or Mach I; they are included below under hardtop and sportsroof, respectfully.
Anyway, here's the breakout from an earlier thread by vamach1:
1972 351HO
19 hardtop
13 conv
366 sportsroof
Ray
Yup, I know. This was a special order...a very special order from Ford. The engine is the real McCoy.
Many of the R code Mustangs for 1972 ran the Boss 351 fender decals.
The owners as well as many salesmen identified these cars as Boss 351's -but they were not. While they did carry the closest relation to the actual 1971 Boss 351's, they were 1972 351 HO equipped Mustangs.
It would be very interesting to see the motor and verify the parts of this special order motor as purchased by the dealer you reference. It is apparently a very rare 1972 351 HO Mustang.
I have always appreciated all of the 1972 HO Mustangs and have recognized them as very desirable cars.
Ray