The rare A mark porcelain cup to be offered by Woolley and Wallis in June.

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Only around 40 pieces of 'A' mark porcelain have so far been discovered, making this group of mid-18th century tablewares qualify for the 'rare as hens' teeth' epithet.

Current ceramic scholarship attributes these pieces to the Bow factory.

Anton Gabszewicz, the scholar and writer on the Bow factory, thinks they were made at Bow's earliest factory around 1744 which would pre-date Chelsea's first products and make them the earliest English porcelain.

With most of the identified pieces now in institutions, the little cup shown here which came into the Salisbury rooms earlier this year on a valuation day is a new addition to the A mark corpus.

While predicting demand for something so rare is not an exact science, Woolley's ceramics specialist John Axford has some precedent to go on.

In October 2002 Dreweatt Neate sold a very similar cup for £9000 and the following year Christie's sold one for £8500.

Both the other cups carry marks and while the Salisbury cup is not marked, its similarity to the Dreweatt's example was one of the main reasons for Mr Axford's atribution. His guide price is £3000-5000.