Same here.. Mine I believe is from the late 50's.. The thing gets groups like yours and I absolutely love it.. Also cycles like butter and is so smooth and broken in..I wonder what kind of accuracy people are getting from their 336 rifles. Mine is an older / gunshow one with a clunky weaver side mount. But it shoots right around an inch at a hundred, sometimes better.
My favorite loads are the Federal Premium 170gr Nosler Partition, Winchester 190gr Silvertip, and Remington 150 grain Core-Lokt.
Got a 336W myself, Good shooter. If I am correct, the Marlin 30s have a front sight without the hood and a birch stock, but are the same action as a 336. The 336W is made for Walmart if I remember right.