Agree totally with Ron, albeit the S prefix was always attributed to the M81 with an R going onto M82's but I know in conversations I have had with Bill Smallwood about that, he states at the "end of Parker Hale" under Modular Industries, they had to use as many parts as they could to complete rifles and little or no creedance was paid to a letter, just get the rifles sold and out of the door policy.
Concur with the views it is definately an M82, of which I can find no trace in the PH files, and what I said earlier perhaps reflects that. The workforce were lucky to have been retained, and thereby morale must have taken a serious dive.
M82 was indeed stamped Special Proof 20 Tons, so that is correct and confirms it for me
The M81 Classic rifle was a Sporter, but the actions were exactly the same, so you can see how easily it is to get confused.
Great buy mate enjoy your M82 and of course the Easter weekend. Stay Safe