The Cotman Letters 1834

The undated autograph letter to an unnamed correspondent was sold at Sotheby’s March 23, 1936 (lot 156) & bought by me.
On the back in JSC’s calligraphy is
4 | 1.5.0
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6.3
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18.9
which suggests a calculation of a 25% discount (which he allowed to the trade & to his fellow artists) on an etching, perhaps?
As he speaks of his long stay in London, the letter must have been written during this fortnight when he returned to Norwich after his appointment at King’s College. He had been in London for a month or more, canvassing.