Chris Cordes has provided this cover
The visit by the British Association for the Advancement of Science included a period in Johannesburg between 28th August and 1st September 1905. Postal facilities were set aside for members to use during their stay. Included in the postal arrangements was a suitably inscribed canceller, as seen on the cover illustrated below. To date, I have only seen either ½d or 1d values of Transvaal stamps used on such covers or postcards. This is in contrast to the coverage in Bloemfontein and Victoria Falls which generated substantial philatelic interest.
Has anyone seen other Transvaal values carrying the BA Johannesburg postmark, either on or off cover?