Hello Michael
The usual tunic button for the Royal Wiltshire Yeomanry had a crown over the Prince of Wales's plumes and motto or a second type with 'RWY' below the plumes. If you have Ripley's book on Yeomanry Buttons then see figures 228 and 229. A cap size button (actually on a cap) believed to be for the Wiltshire Yeomanry was shown in a recent thread -
http://www.britishbadgeforum.com/for...9&d=1505084329
The problem is that cap buttons were often quite different from the full size tunic buttons so recognition can sometimes be difficult.
Furthermore, many regiments (including some in the British Indian Army) used the Prince of Wales's plumes and motto and your cap button could belong to any of them. Being silver it could be for a Volunteer Battalion of an infantry regiment. Difficult to be sure because so little has been published about cap buttons.
Roger