I think they are for adjusting the guns by use of a periscope.
You place the target the set distance away then adjust the alignment of the guns by using a periscope attachment.
The test firing was usually into something similar to this..
I have a picture in a book of an armourer using the periscope on a spitfire but the photo dosn't show what he's 'aiming at'. It is being done out on the airfield in the photo so there would have been no actual test firing. Just aligning.