This design is of a 1st Australian Horse, N.S.W. 1897-1903 uniform button. Cossum records gilt and brass versions on page 6 of his book, but this is whitemetal. (There was a blacked bronze version was sold by Noble Numismatics.)
As a reporduction, it is “not qite right”. It is noticeably heavier than all my other uniform buttons, in fact so heavy that I think it is solid. Also, it has a different shank; a drilled through wedge rather than the fast or drop wire shanks on other buttons of the era. There are two circular ‘punch marks’, and a strange stippling to the surface. The most interesting thing, however, is the backmark. Instead of Hobson & Sons 1,3& 5 Lexington , it is mis-spelt Hopson & Sons.
Hobson & Sons have been uniform makers (not button makers) since 1850. According to their website ( http://www.hobsonuniforms.co.uk/aboutus.htm#howdidwe ), they acquired a shop in Lexington Street around 1880 (although some collectors quote 1887). Presumably Hobson & Sons would commission buttons for the uniforms they made from one or more button manufacturers.
The 1st Australian Volunteer Horse were gazetted in NSW on the 1st August, 1897.
This cavalry served during the Boer War and was the source for a number of recruits for the new Australian Flying Corps during World War 1.