"Whatever are those tatty old bits of metal?" I hear you cry.
Those, dear friends, are the result of months and months of work when I was 16-17 and they made me cry.
I started an apprenticeship in the civilian workshops of the Royal Electrical and Mechanical Engineers.(REME)
After learning about hand tools and how to use them we had various test pieces to make.
This was one of them. It looks tatty because it has been wrapped in wax cloth for 41 years.
Two pieces of ½" thick mild steel plate had to be cut to 4"x3" and 1"x1" exactly.
Then a 1"x1" hole was cut in the 4"x3" so that the small square would fit in tightly all eight ways. So everything had to be exact and square.
A surface plate, engineers blue and India stone was used to finely remove high spots.
The Foreman would check the work. Twice he threw mine in the bin and said " I could drive a bus through there, start again."

Not a happy time.