The following pictures are of the Hi-Speed and standard velocity main spring housings found on first series Colt Woodsman pistols. The parallel lines on the left are for hardened housings that allow the use of Hi-Velocity cartridges. The oval checkered pattern on the right indicates standard velocity cartridges ONLY!
One need only be concerned with this distinction on first series Colt pistols; i.e., pre-Woodsman and Woodsman pistols made before WW2. Phase-in of Hi-Speed housings began in 1932 at about serial number 81,000 and by 86,000 all had the Hi-Speed main spring housings. Some first series pistols were assembled after WW2 until about 1947 but they were all made with Hi-Speed housings. This information comes from Bob Rayburn's "Colt Woodsman Pocket Guide.