Here is a cover from Johan Bezuidenhout.
Schoemansrust post office is being recorded in Put as open from 1890-1898. While Put did not record a postmark, there is one in Alex Vissers’ addendum dated 5 May 1899. This cover, cancelled 6 June 1898 and sent to Sea Point, C.G.H. (received 8 June, with two different Potchefstroom transit marks dated 7 June ) could possibly be the earliest recorded postmarked date, and maybe even the first on cover too.