I suspect I'm not the first to do this. I had a bottle of Old Holland cold pressed linseed with a childproof cap that broke and there was no way that stupid large piece of plastic was coming off. I put it to one side for a few months, then decided I may as well try breaking it off to get some use of it even with a reduced life span. I slowly cut it off with a bonsai tool of all things. I was surprised to find that the bottle thread looked very conventional. Why I was surprised seems odd itself. It can't have been the first time I saw the thread, but I guess I never inspected it.
Took a cap off an empty bottle of wine, (Here in New Zealand our wine bottles are pretty much exclusively resealable caps. They tried composite cork plastic cork and caps. the caps worked best.) rinced it with water, dried it, put it on the Old Holland bottle. It fit perfectly. Tried the Winsor & Newton bottles, also perfect. I decided that it would be best to use the ones with silvered inner seals instead of the white plastic inner seals because that was the type used inside the original medium bottles. It's been about a year now and no strange reactions. I have no small children in my care so have no safety quarms about doing this. The bottles don't jam up with medium at all, open easily, and even look much better. Whenever I get a new medium bottle now, the childproof goes straight in the bin.
Took a cap off an empty bottle of wine, (Here in New Zealand our wine bottles are pretty much exclusively resealable caps. They tried composite cork plastic cork and caps. the caps worked best.) rinced it with water, dried it, put it on the Old Holland bottle. It fit perfectly. Tried the Winsor & Newton bottles, also perfect. I decided that it would be best to use the ones with silvered inner seals instead of the white plastic inner seals because that was the type used inside the original medium bottles. It's been about a year now and no strange reactions. I have no small children in my care so have no safety quarms about doing this. The bottles don't jam up with medium at all, open easily, and even look much better. Whenever I get a new medium bottle now, the childproof goes straight in the bin.