The days of 24/25 March 1976 must have been quite a time at Roberts International Airport: likely the only time in Liberia's history that the airfield was visited by a supersonic jet (although the craft was not unknown in West Africa, as Air France also ran a Paris-Dakar-Rio de Janeiro service for a time).
Its not clear from this jubilant cover whether passengers were even on board, much less if any deplaned at Monrovia before roaring on to Cape Town. From the red cancellation stamp, it suggests that it was a corporate test flight for the British Aircraft Corporation (BAC), and the flight clearly originated at Fairford, a town in the Cotswolds which is home to an RAF base, as opposed to Heathrow. Robertsfield was constructed with an especially long runway, which made it an appealing stopover for jets from Europe to the southern hemisphere (and also made RIA an alternate landing site for the NASA Space Shuttle).
This celebratory envelope does have a small chart comparing standard flight time of more than 5 and a half hours with the Mach-speed duration, less than 3 hours elapsed from Gloucestershire to the Guinea coast.