Interesting critter, whatever it may or may not be. I would presume the surest way to prove out the YB is to buy a male Ivory, breed them, and see if any more Ivories pop out. If not, than you at least know that the offspring are all going to be YB something.
I also looked at some Pewter Bellies on Google and I'm curious to know what their bellies look like (if their are any folks who have one, I think belly shots are in order) because they seem to have clear stomachs to me but I've yet to see so who knows. ¯\_(ツ)_/¯