Hi Dillon,
Parker and Waterman converters, although made in the same factory, are NOT compatible, or the same for that matter.
The only converters compatible with each other are international size converters, although different brands do not necessarily fit into each others barrels.
On some Waterman pens an international converter will work, like the Pelikan one. I am thinking specifically of some of the early CF pens (yes, here is the replacement converter for those hard to get original ones

, provided the barrel is wide enough to accommodate them; use a single layer (yes, 1 and 1 only!!!) of teflon tape around the bottom part, not covering the nipple, to make it seat sturdily), and some of the newer models, with barrels wide enough to accommodate them.
The nipple shape of Parker and Waterman converters is not the same, and neither is the diameter of the hole, or the receiving end in the section for the nipple.
I'll post a pic if I manage to get around to making one in the first place, this weekend (I do have to work, unfortunately, so I'll see how things work out). I can't show a pic of a Rotring converter, though: I simply don't have one: I am not into Rotring pens. I can show a few Lamy converters instead

.
Warm regards, Wim