-SG- ... when it's a South Australian Railways copy!
Many thanks to Leon for a profusion of photos - and information - regarding the 557cc SAR engine dating from about 1910 to 1915. It seems they were fitted to small railway maintenance trucks and quite a few of them were to be had back in the sixties and seventies. But now, they are pretty rare.
Although the crankcase castings incorporate substantial extra mounting lugs, it is very clearly a Norton copy. Whether Nortons were paid any sort of premium for use of their basic design is not known. The photos of the cylinder barrels - Norton on the right in all cases - shows the design similarity and the cams/followers look much like those employed during the twenties.