Just being curious, but has your SMLE got the magazine cut off???
My SMLE Mk III* 1917 doesn't have a magazine cut off nor any slot for it which I understand was on the MKIII but omitted on the Mk III* but it does have 'windage adjustment' on the backsight so is a bit of a hybrid between MkIII and MKIII*. I took advice from this forum when I wanted to buy one few years back and followed the advice and finished up with a really nice SMLE with lots of 'patina', knocks, a couple of strange threaded holes in the woodwork which may have housed an extra sight (?), also marked with the Union of South Africa mark which is of interest to me because I visit Deville Wood every year as part of my annual study of the Somme area near it.
Thank you all for the information about my sten - very interesting.