An "R" prefix would make it a 1958 production rifle. I think that $150 for any 336 in better than 90% condition is a good buy and the fact that I love the Sporting Carbine configuration would make it even better for me.
Your rifle is the 2nd style of 336SC and should have the following attributes:
1. 20" barrel with Micro-Groove rifling.
2. Pistol grip cap with white line spacer and white line spacer under the buttplate.
3. Was not d&t at the factory for a top mount scope.
If you find a 336 SC in 35 in the same condition and for the same price, I'll buy it from you.
Jeff
