1643 Charles I Oxford Mint Halfcrown - UNIQUE UNATTESTED "A" MINT ?

Charles I (1625 - 49), Oxford mint (1642 - 46), Halfcrown, 1643 (first period), m.m. plume, obv., letter A added in field below horseman after striking, apparently to represent an issue of the unattested 'A' mint which became active in 1645,  (Morr. B-3 and plate III, 34 (this coin); Bull, pp. 109-110, 599 (this coin)) - weight 15.05 grams

Unique, Morrieson wrote 'I am unable to propose any explanation of this A'. 

PROVENANCE

Ex Mrs Street

Ex Rt. Hon. R.W. Marsham, Sotheby, 19 November 1888, lot 558, part;

Ex H. Montagu (probably a private sale); 1896;

Ex J.G. Murdoch, Sotheby's, 8 June 1903 (lot 170),

Ex Spink's Numismatic Circular, October 1905, (20426),

Ex G. Hamilton-Smith (I), Sotheby, 21 July 1913, lot 89;

Ex R.C. Lockett, part VII, Glendining, 1-6 November 1968, lot 3470.