Tin coinage

Initially irregular in shape, by the 19th century these blocks became standardised as bevel-edged rectangular cuboids weighing 3.34 long hundredweight (170 kg).

At the stannary town the prover, a duchy officer, struck off the corners of the blocks to check the quality of the metal.

[2] The earliest surviving record of the coinage dates to 1156 when Richard de Tracy had permission to collect the revenue for the following four years.

A 1305 charter of Edward I[7] identifies the towns of Lostwithiel, Bodmin, Liskeard, Truro and Helston as locations for proving Cornish tin.

By 1837 the proving took place only at Morwellham in Devon but at Calstock, St Austell, Truro, Helston, Hayle and Penzance in Cornwall.

The group stated that this was the amount by which Cornwall had been overcharged on tin production compared to the rate of taxation in Devon between 1337 (when the duchy was founded) and 1837 (the final year the tax was collected).