Royal Mail Elusive Mount Pleasant Sorting Office Cover. Issued to commemorate the 125th Anniversary of the GPO Mount Pleasant Office, officially opened on 30/8/1889, on the site of Coldbath Prison. Once acknowledged as the largest post sorting office in the world, it continues to handle millions of postal items daily. Only 100 of this important cover were ever produced. Bearing the iconic Machin stamp postmark. Dated 30/8/14. Offered in superb condition.