In 1873 he bought the British Guiana 1856 1c magenta stamp for six shillings from a local schoolboy, Louis Vernon Vaughan.
Vaughan had found the stamp among the effects of his uncle Andrew Hunter who had recently moved to Barbados.
[10][11][12] In addition to the 1c magenta, McKinnon owned all four known single used copies of the 1850 2c "cottonreel" stamps on rose-coloured paper.
[11] In 1878 McKinnon sent his British Guiana collection, included the 1c magenta, to his friend Robert Wylie Hill in Glasgow for sale.
[11] The collection was then offered by post to several other dealers and was bought by Thomas Ridpath for £120[10] who soon sold the 1c magenta to Philipp von Ferrary.