Cam - I discovered, after some research, that Britstand was purchased by the Clyde Group in 1958, and became a subsidiary of Clyde Industries, one of Australias largest, and amongst the most famous, engineering/manufacturing/finance groups in the nation.
After Clyde took over, they regularly referred to Britstand (British Standard Machinery) as BSM.
Thus, you're correct in your assumption that the casting is a Britstand casting, manufactured when Britstand had already been a Clyde subsidiary for a number of years.
Another interesting thing that I found was that Aveling-Barford, the Pommy earthmoving equipment manufacturer, bought a large slice of Britstand in 1955, when Britstand started running into financial difficulties due to poor sales, and when they needed a capital injection.
I am not sure when A-B got out of Britstand, I'm presuming that Clyde bought up A-B's shares in Britstand, when they took over in 1958.
Britstand did well during the 1930's, during WW2, after WW2, and up until 1952 .. the year when they recorded a record profit and paid a 10% dividend.
However, that was their last good year, and things went downhill from there, and they never paid another dividend again, due to poor profit levels, until after Clyde took over.