NRS 202.3653 defines a concealed firearm as "a loaded or unloaded pistol, revolver or other firearm which is carried upon a person in such a manner as not to be discernible by ordinary observation." NRS 202.350 prohibits carrying a concealed weapon without a permit. You can read the NRS on firearms here: https://www.leg.state.nv.us/NRS/NRS-202.html
My layman's breakdown: If I can look at you (aka ordinary observation) and tell you have a firearm, it is not concealed. If it's not concealed, it's legal. There is no definition of open carry. Furthermore, my personal opinion is that the flap top holsters are totally OC/non-concealed. No one ever looks at a flap top holster and say "Oh, I wonder what that is..." No. They don't. It's a holster with a gun in it. Again, just my opinion.