I have one mano that I found at the out cropping of a mountain that they are harvesting the volcanic rock from for grills, road rock etc. This mano has holes in it larger than 1/4". I think they grabbed whatever rock felt comfortable for them to handle and went to grinding.
Again, hold it up to the light and if it feels smooth, looks warn and is shiny it is probably a mano. As mentioned it is hard to tell from the photo.
I use an Eastwing rock hounds staff that has a clam shell at the base. I think I flip over more rocks out there than most. I quit picking up the single sided mano's and just collect the two sided ones. I have probably given half of my collection away and still have over 60 of them.