I believe Marlin started drilling and tapping for scope mounting about 1954 1/2. I bought a 1953 336 SC off Gunbroker in 2007 in 97% condition which had not been drilled and tapped ever with the Ballard rifling. The "Sported carbine" models have a heavier less tapered barrel than the 336-A models or the full-length carbines which made the rifle to ME..front heavy. I sold the rifle to a fella in North Dakota who wanted the rifle for cast bullet shooting. I replaced the sported carbine with a 1953 336-A in .35 Rem with the longer 24-inch tapered barrel. The rifle shoots great as long as you don't shoot Remington "Core-Lokts" either factory ammo or hand loads through it. I consider the Marlin 336-A's the POOR Man's model 64 Winchester.
That agrees with what I've found. Had a 1955 SC with Ballard rifling that was drilled and tapped, but in looking around the net the ones made before 1954 and some made in that year as well were waffle top with no factory d&t.
