Your mark is exactly the same as the one for William Kerr shown on page 135 of the "bible" - Australian Jewellers Gold & Silversmiths / Makers & Marks by Cavill et al. His dates as an independent jeweller were 1875 until his death in 1896, and the firm William Kerr continued by his 3 sons until closure in 1938. Although best known as medallists, particularly Masonic, and as makers of fine presentation silver, the firm apparently stocked a large quantity of retail jewellery - I would date your brooch 1890-1920. It does not look to be of very great quality and may well have been a line that was simply retailed by the firm. But an interesting mark, and one that I don't have in my jewellery collection.