This is a trial lens kit for bifocal lenses, sold by Western Optical in St. Louis, Missouri.  The lens rim and handle are the standard diameter which fit in a trial frame.  The crescent area around the small, circular lens has no glass.  The clever difference is that this kit includes an early 1900's version of a bifocal segmented lens.  At the turn of last century, a small lens segment would have been glued or fused into the distance lens to create a bifocal.  This kit allows the optometrist or optician to demonstrate the bifocal effect with trial lenses, before grinding the lenses.  Two trial lenses are missing, but considering that the patent is from 1909, this could be a nice addition to someone's collection.