Maybe the double gunners have the definitive answer:

http://forums.accuratereloadin.../4711043/m/217102289

Mike:

What you have are bullets made by the original Kynoch company in the early '70s or earlier, not the current Kynoch company. If the jackets are gilding metal (copper), they're from the post-war era (Kynoch halted manufacturing in 1973). If they are nickel, they're pre-war. Simple nickel jacketed softs and solids both seem to have worked fine. After WWII, it wasn't feasible to continue using nickel due to costs and shortages, so the jacket material was changed to simple gilding metal. Both softs and solids failed miserably. As a result, gilding metal-covered steel jacketed solids began to appear about 1951 and, in some calibers (including .500 Nitro Express), gilding metal-covered steel jacketed softs followed. A magnet will tell you if your softs are steel reinforced, or simple gilding metal.

Original (Cordite) Kynoch always had the cannelure at the base of the bullet, because they never crimped in the way Americans do. They relied primarily on tight neck tension (smaller neck ID before seating bullet), plus a stab crimp (usually 3 point) at the base of the neck where the low cannelure you're seeing would be with the bullet seated.

Hope this answers your questions.

Thanks Mark. That explains it. I will check mine tonight with a magnet to see if they are steel reinforced.