Alastair
As you found it in your workshop I think it's appropriate that the front looks like it has been used as a hammer at some time. That said I would still take it over the vast majority that you see on offer. From the front the things that look good for me are -
1. The spacing between the R and the top left hand corner of the turret, on the copies the R practically touches the turret but on the originals there is a distinct gap.
2. The thickness of the RNAS lettering, on the copies it tends to be a skinny font but on the originals it is noticeably thicker.
And on the back
1. The positioning of the lugs, on the copies they tend to be very close to the left and right hand edges but on the originals they tend to be positioned above the wheels.
2. The spokes of the wheels give that nice puckered appearance
Paddy