I think that the holland system can be done with a coin if IRC.

I don't have one to look at but I know the big screw definately can, I just can't remember whether the rear screw is also bigger than normal and has a bigger slot.

Of the threaded barrel or the H&H system, I prefer the H&H system, even though it is not as short as the threaded barrel system, it allows scoped rifles to be taken down without having to resight the gun in.

What is the NEWTON system - could you explain / provide
a link to pictures ?