Don't quote me on this, but IIRC the different lengths of 18.8mm joints themselves are:
US 22mm (19/22)
ISO (Euro) 26mm (19/26)
This is just the length of the ground joint itself (or polished joint), not the whole stem, adapter, whatever. A small difference, but can be needed depending on what you're using it with. From reports I've seen here on FC, Zion seems to prefer the shorter US (19/22) joints so the bottom of the joint doesn't hit the screen at the bottom of the chamber.
Pretty sure the slope of the sides are exactly the same. So, if you cut the last 4mm of an ISO joint you have a US joint. If you have the necessary 4mm clearance at the bottom of the female joint you're connecting to, it wouldn't matter which.
Hope this makes sense. I know what I mean, but hope all get the gist of it.