Good luck with your quest, xtratoy. I had a pair of those gadgets back in the late 60s and early 70s. IIRC, there's no mystery; but, after all the years, I suspect the O-ring may have lost some of its elasticity and want replacing. The same precautions that apply to modern full length tubes also apply to the four-tenners, clean the barrel and the four-tenner before installation and center the four-tenner's extractor over the gun's extractor.
The only problem I had (other than marginal pattern development) was when I didn't keep the four-tenner's extractor in contact with the gun's while loading. (The club where I used to night shoot wasn't well lighted at the shooting stations.) If the four-tenner's extractor is pulled out beyond the gun's, then there there's the possibility that it will deflect while loading and allow the head of the .410 round to get below the little extractor. If you keep the two extractors in contact, then the hull can't sneak under the top one.
