Yes, it's not thaaaat confusing:
Splinter = 19/19
Xl8r = 19/19
Milaana3 = 19/19
SplinterZ, Zion, Milaana1,2 = 19/22
19/19 is an RBT custom size, shorter than the 19/22 standard, all 19/19 male stems fit 19/22 female bowls, just not vice versa...
If you just have Splinter V2, you have the custom RBT 19/19 size female glass so it means any current RBT stems you find are going to fit, I don't know anywhere you can find the old 19/22 RBT stems, they are all stocking only 19/19 stems now... You can get custom-made stems, and wooden stems, that are 19/19 as well... The first number is the width of the joint, the second number is the length of the joint, so any 18aka 18.8aka 19mm joint can be cut down to fit, most common standard is 19/26...