Mystery metal plates (Fulbourn CB21)

4 tatty steel plates 8" x 8½" or 200 x 215 mm each with a square hole punched near the middle, and 4 long coach bolts and nuts to fit the holes. Bolts are 221 mm or 8¾" long. If you have the slightest idea what these are, you are welcome to them. Possibly for retaining a big pad?

       
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