This object is a regimental badge of the Middlesex Regiment (Duke of Cambridge's Own), which commemorates the Battle of Albuera. The regiment received the "Albuhera" battle honor for its actions during the Battle of Albuera in 1811, during the Peninsular War. The 57th Infantry Regiment (West Middlesex), which later became the 1st Battalion of the Middlesex Regiment, participated in this bloody battle, which earned it the nickname "Die-Hards".

Details of the badge:

Design: The badge presents the feathers of the Prince of Wales in a crown, with the motto "Ich Dien" (I serve), above the crown and the figure of the Duke of Cambridge. Everything is surrounded by a laurel wreath, with the inscription "Albuhera" and "Middlesex Regt" on parchments below.

Use: Although it is a standard military cap badge for the regiment, the image you provided shows a version mounted on a ring, suggesting that it may have been worn as a medallion, pendant or watch fob.

Period: Badges of this type were worn from 1898 to 1958, with different metal versions produced during wartimes.