Solitaires, or bachelor's buttons, were made in the 1870's-90's to avoid the bother of having to sew on a button, or the struggle of forcing a stud through the thicknesses of material.  By means of their removable back, they could be fixed in an instant.  Though the mechanism can vary,
almost all have the wings on the side of the top which release the back.  Sizes vary from 3/8" to 1" and are classified by the National Button Society  as Division IV Section 6-1 or 6-2.  There is a fine article about these beauties in the British Button Society  publication Button Lines,
#57 December 1988.

Visit the British Button Society to inquire about the complete article.  Or CLICK HERE.