This copy of the postcard indicates that a 1/2d stamp is required. The postage rate for postcards rose to 1d in 1918, so the image presumably does not date from later than that year. The mill is on the River Teme at Downton Castle, Herefordshire.
Account for oats and bran supplied in September and October 1887 by W.Terrett, Miller and Corn Merchant, Heol-y-dwr Mill and Cusop Mill, 18 and 19 Broad Street, Hay to T. Dew Esq, Whitney. Account settled 18 September 1888.