28-2. SN N502346.
I would bet those target stocks came with it from the factory. It's late enough in the run of frames that it may have shipped around 79 or 80 with them.
I have that exact 28-2 and it's a dream to shoot. Even 158gr Rem SJHP full power loads are easily handled with that big gun. Accurate too of course.
Great find - It's a classic!