I wasn't sure what this was about until I found a reference to it in this extract:
"I was to be based at “HMS Drake”, as a probationary Engine Room Artificer (Number D/MX 74662) and await the results of a ‘trade test’, in order to ascertain whether I had the necessary high standard of skills, to be accepted into that particular engineering branch. There followed a week of hard work at the bench, using hammer and chisel, various 12 to 14 inch files, from rough to smooth, scrapers etc. to complete the test piece, which was a ‘Strap and Block’ and my efforts resulted in the development of very large blisters on the palm of my right hand. The accuracy of the finished product was to be finally checked on a ‘surface plate’ using a ‘clock gauge’. However, I passed my acceptance test with a comment from the Engineer Captain, “Excellent piece of work Ball, but you will have learn to work much faster!” From then I was officially an Acting E.R.A. 5th Class."