Originally Posted by Vort
The first several years after receiving my endowment, I wore one-piece almost exclusively. There was a slit in back that you could pull open to either side to take care of restroom business, but otherwise there was no "hole in the bottom". It was very much like a Union suit, but instead of a button flap, it had the overlapping slit. Without having actually seen what you saw, I can't really comment on that.
If you look at the women's one piece styles at store.lds.org, the Snug Leg style advertises an "open crotch" while the Flare Leg style advertises a "closed crotch."
I've never seen them myself, so I can't comment more than that.