Does the mystery model have a date code on it? And if so, is it close in year of production to the other two models?
This idea probably won't get you any closer, but - since you have an ASM Navy and a Pietta Navy to compare it against, I would be tempted to see how the grip geometry compares (backstsrap & triggerguard) from the mystery model to the other two, would it exchange and fit with either of the others, or maybe it's close to fitting on one but not even close on the other, etc.
Whether it fits or not, I still don't think it would prove anything definitive as even amongst the same manufacturer's guns the geometry often differs enough to not allow parts like that to interchange, but as part of solving a mystery, it'd be something else to test...
Whip