Im gonna go out on a limb here
I think when Palitoy took over the line from GI Joe they used the name "Soldiers of the Century" here in the UK and released the German Stormtrooper, British Infantry, French Resistance, Australian Jungle Fighter (kahki Version,Russian and Green Beret. The same range as GI Joe had out except for the Japanese Imperial Soldier. So to show it was essentially a different range they used the name "Soldiers of the Century"? This was around Late 68-early 69
Then When GI Joe discontinued their Line of "Soldiers of the World" Palitoy changed the range to "Soldiers of the World" here in UK and reissued the German Stormtrooper,British Infantry and French Resistance outfits but added "Australian Jungle Fighter" (Green Version) French Foreign Legion plus The German Staff Officer, and dropped the khaki Jungle fighter, Russian and Green beret which were as far as i know NEVER released as "Soldiers of the World" here in UK so the name change was a launch pad for the 3 new outfits in 1970 knowing that now they were discontinued in the USA there would be no Mix up as to what outfit variations there were to be.
Hence the rename and relaunch
Thats my theory anyway
Im sure our friends across the pond can confirm that the name "Soldiers of the Century" was never used by GI JOE
Great Post Steve how refreshing to have an Action Man/GI Joe question to chew over