A nickel safety stirrup.

The stirrup has a swinging tread and arch that splits open to release the foot in the event of a fall so that the rider cannot be dragged.
It has an open tread, curved at its front edge and stamped "HEEL" at its rear edge - if fitted the wrong way round the escape mechanism would not work.
The stirrup was much used - the tread has thinned and worn smooth on its front edge.
The underside of the footplate or tread is stamped "Scott's", "Safety" "Size 4" , "H & S", "Solid Nickel" and "73277"
Dimensions : 6½" (16.5cm) tall x 5" (12.6cm) external width x 1¾" (4.6cm) deep tread
Internal width 3½" (9cm)
Weight 1lb ½oz, 470g