The Colt Model 02 also known as the 602 was the first to use the 1 in 12 barrel twist (the Model 01 used the 1 in 14 twist). The civilian Colt SP1 was very close to the Model 602 and was available in 1964.

In the book "The Black Rifle" page 305 is a photo of a Colt Model 01 rebarreled in the UKicon to use the 4.85x49mm cartridge

Photo shows a 4.85mm,Attachment 40641 224E2 Winchester and 222 Remington Special